keventd: record the libblkid build requirement

keventd is the only thing in the tree that links libblkid, so a tree
that used to build now stops in configure with no hint of which package
to install.  Say so where people look for dependencies, and give CI the
package it now needs.

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

doc/build.md

@@ -8,6 +8,13 @@ optional plugins to enable.  It depends on three external libraries:
 - [libite][] (-lite), much needed frog DNA
 - [libConfuse][], the parser behind the .conf format
 
+The bundled device manager, keventd, needs a fourth: libblkid from
+[util-linux][].  It reads the filesystem UUID and label off block
+devices, which is what the curated rules name the `/dev/disk/by-uuid/`
+and `/dev/disk/by-label/` symlinks after.  keventd is enabled by
+default, so this is a hard requirement unless you build with
+`--without-keventd`.
+
 > [!IMPORTANT]
 > Most free/open source software packages that use `configure` default
 > to install to `/usr/local`.  However, some Linux distributions do no
@@ -24,6 +31,7 @@ version of the above mentioned libraries.  Currently required versions:
 - libite v2.2.0, or later
 - libuEv v2.2.0, or later
 - libConfuse v3.3, or later
+- libblkid, any version, when keventd is enabled
 
 
 Configure
@@ -67,6 +75,10 @@ Below are a few of the main switches to configure:
 
 * `--enable-x11-common-plugin`: Enable the optional X Window `x11-common.so` plugin.
 
+* `--without-keventd`: Drop the bundled device manager, for systems that
+  run mdev, mdevd, or udevd instead.  Enabled by default, and the only
+  thing that pulls in libblkid.  See [Bundled Device Manager](keventd.md).
+
 * `--with-sulogin`: Enable bundled `sulogin` program.  Default is to use the
   system `sulogin(8)`.  The sulogin shipped with Finit *allows password-less*
   login if the `root` user is disabled or has no password at all.
@@ -259,3 +271,4 @@ it only for debugging start up issues when Finit crashes.
 [libuEv]:  https://github.com/troglobit/libuev
 [libite]:  https://github.com/troglobit/libite
 [libConfuse]: https://github.com/libconfuse/libconfuse
+[util-linux]: https://github.com/util-linux/util-linux

doc/keventd.md

@@ -6,8 +6,11 @@ with Finit.  It replaces the need for external device managers like
 mdev, mdevd, or udevd on systems where a lighter-weight solution is
 preferred, particularly on embedded systems.
 
-It is enabled by default since Finit v5.  To disable it and use an
-external device manager instead:
+It is enabled by default since Finit v5, and needs libblkid from
+util-linux (the `libblkid-dev` package, or whatever your distribution
+calls it) to read the filesystem UUID and label behind the
+`/dev/disk/by-uuid/` and `/dev/disk/by-label/` symlinks.  To disable it
+and use an external device manager instead:
 
     ./configure --without-keventd
 
@@ -103,6 +106,10 @@ among others:
 They also set up a few fixed names: `/dev/rtc`, `/dev/cdrom`, and
 `/dev/virtio-ports/<name>`.
 
+The `by-uuid` and `by-label` links are why keventd needs libblkid.  The
+rules read the filesystem metadata off the device with
+`IMPORT{builtin}="blkid"`, and libblkid is what that builtin calls.
+
 Built-in and rule-provided symlinks alike are tracked internally and
 removed when the corresponding device is unplugged.
 
@@ -252,8 +259,6 @@ Usage
       -G        Disable netlink rebroadcast entirely
       -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