keventd: add udev rules engine

Transform keventd from a power-supply monitor + basic hotplug handler into
a full udev-compatible device manager.

Rules engine (rules.c):
- Full .rules file parser covering all udev key types: ACTION, KERNEL,
  SUBSYSTEM, DEVPATH, ENV, ATTR, SYSCTL, TAG, RESULT, PROGRAM, TEST,
  parent-chain KERNELS/SUBSYSTEMS/ATTRS/DRIVERS, and more
- Pattern matching: plain string, fnmatch glob, and pipe-separated alternatives
- Operators: ==, !=, =, +=, -=, :=
- Assignments: NAME=, MODE=, OWNER=, GROUP=, SYMLINK+=, ENV{k}=, TAG+=, RUN+=
- IMPORT{program|file|builtin|parent|cmdline|db}=
- PROGRAM= with stdout capture for subsequent RESULT== matching
- GOTO=/LABEL= flow control
- Loads *.rules from /lib/udev/rules.d, /run/udev/rules.d, /etc/udev/rules.d
  and an optional extra directory (-r DIR); reloads on SIGHUP

Builtin framework (builtin.c):
- kmod:     load module by MODALIAS or explicit alias
- hwdb:     match device against *.hwdb text files in udev hwdb dirs; builds
	    correct lookup key per subsystem — evdev:input:b*v*p*e* for input,
	    usb:v*p* for USB, raw modalias for PCI/platform
- path_id:  build stable ID_PATH / ID_PATH_TAG from sysfs topology (PCI, USB,
	    ATA, NVMe, platform, ACPI, virtio)
- usb_id:   read idVendor/idProduct/bcdDevice/serial from sysfs; look up
	    ID_VENDOR_FROM_DATABASE and ID_MODEL_FROM_DATABASE from usb.ids
	    (hwdata package) when available; silent fallback when absent
- input_id: classify input devices (keyboard, mouse, joystick, touchscreen,
	    touchpad) from evdev capability bitmasks in sysfs
- net_id:   generate predictable names — MAC-based enx<mac> and PCI-slot-based
	    enp<bus>s<dev>[f<func>]
- blkid:    probe filesystem type, UUID, and label via libblkid; sets ID_FS_*
	    and ID_PART_TABLE_* properties

Network interface renaming (uevent.c):
- netdev_add() renames interfaces via SIOCSIFNAME when a NAME= rule matched,
  then sets the Finit dev/ condition on the final name; and any setup using
  persistent interface naming via udev rules

Device node and symlink improvements (uevent.c):
- NAME=, MODE=, OWNER=, GROUP= overrides from matched rules applied at
  mknod/chown time, falling back to the built-in permission table
- SYMLINK+= links from rules applied alongside built-in by-id/by-path links

Device property database (udevdb.c):
- Persist per-device E:/S:/I: records to /run/udev/data/ on ADD/CHANGE,
  delete on REMOVE; IMPORT{db}= restores saved properties into event env

Build:
- libblkid (util-linux) is now required for keventd

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/keventd.md

@@ -20,9 +20,11 @@ kernel events and handles:
 
 - **Device node creation**: creates and removes `/dev` nodes with
   correct permissions on device add/remove events
-- **Persistent symlinks**: creates `/dev/disk/by-id/`, `/dev/disk/by-path/`,
-  and `/dev/input/by-id/`, `/dev/input/by-path/` symlinks for stable
-  device naming
+- **udev rules**: matches events against rules read from
+  `/lib/udev/rules.d/`, `/run/udev/rules.d/`, and `/etc/udev/rules.d/`,
+  with a curated ruleset derived from eudev installed by default
+- **Persistent symlinks**: creates `by-id` and `by-path` symlinks under
+  `/dev/disk/`, `/dev/input/`, and elsewhere, for stable device naming
 - **Firmware loading**: responds to kernel firmware requests by searching
   `/lib/firmware/` and writing firmware data to sysfs
 - **Module loading**: parses `MODALIAS` from uevents and spawns `modprobe`
@@ -70,19 +72,25 @@ device subsystem and name:
 Persistent Symlinks
 -------------------
 
-For block devices, keventd creates symlinks under `/dev/disk/`:
+Symlinks come from two places.  A handful are built into keventd and are
+created for every device in the subsystem, whatever rules are loaded.
+
+For block devices, under `/dev/disk/`:
 
 - **by-id**: based on the device serial number and model, read from
   sysfs attributes (`/sys/.../device/vendor`, `model`, `serial`)
 - **by-path**: based on the device topology path
 
-For input devices, symlinks are created under `/dev/input/`:
+For input devices, under `/dev/input/`:
 
 - **by-id**: based on the device name from sysfs
 - **by-path**: based on the physical device path
 
-These symlinks are tracked internally and automatically removed when the
-corresponding device is unplugged.
+The rest come from `SYMLINK+=` in the udev rules, so what you get
+depends on the ruleset installed.
+
+Built-in and rule-provided symlinks alike are tracked internally and
+removed when the corresponding device is unplugged.
 
 
 Firmware Loading
@@ -221,7 +229,7 @@ battery:
 Usage
 -----
 
-    keventd [-cdGhnpv] [-g GROUP]
+    keventd [-cdGhnpv] [-g GROUP] [-r DIR]
 
     Options:
       -c        Run coldplug at startup
@@ -231,6 +239,7 @@ Usage
       -h        Show help text
       -n        Run in foreground (no daemon)
       -p        Passive mode: power supply events only
+      -r DIR    Extra rules directory
       -v        Show version
 
 In normal operation, Finit starts keventd automatically via its system