keventd: expand scope to become a unified device manager

Evolve keventd from a power_supply-only monitor into a full device
manager capable of replacing mdev/mdevd on embedded systems.  This
is the first step towards Finit v5.0 where keventd absorbs devmon.

New capabilities:

 - Parse all uevent actions (add, remove, change, bind, unbind)
 - Create and remove /dev nodes with subsystem-aware permissions
 - Create persistent symlinks in /dev/disk/by-{id,path} and
   /dev/input/by-{id,path}, tracked for cleanup on device removal
 - Load firmware from /lib/firmware/ via the sysfs loading protocol
 - Spawn modprobe for MODALIAS events (async, non-blocking)
 - Coldplug support via -c flag (walks /sys/devices to replay events)
 - Set dev/* conditions for Finit's service dependency system

The original power_supply monitoring and sys/pwr/ac condition are
preserved.

New files: keventd.h (structures/API), uevent.c (all device logic).
The receive buffer is increased to 8K with a 1MB socket buffer to
reduce event loss during coldplug bursts.

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

configure.ac

@@ -124,7 +124,7 @@ AC_PLUGIN([resolvconf],    [no],  [Setup necessary files for resolvconf])
 AC_PLUGIN([x11-common],    [no],  [Console setup (for X)])
 AC_PLUGIN([netlink],       [yes], [Basic netlink plugin for IFUP/IFDN and GW events. Can be replaced with externally built plugin that links with libnl or similar.])
 AC_PLUGIN([hook-scripts],  [no],  [Trigger script execution from hook points])
-AC_PLUGIN([hotplug],       [yes], [Start udevd or mdev kernel event datamon])
+AC_PLUGIN([hotplug],       [no],  [Start udevd or mdev kernel event datamon])
 AC_PLUGIN([rtc],           [yes], [Save and restore RTC using hwclock])
 AC_PLUGIN([tty],           [yes], [Automatically activate new TTYs, e.g. USB-to-serial])
 AC_PLUGIN([urandom],       [yes], [Setup and save random seed at boot/shutdown])
@@ -177,7 +177,7 @@ AC_ARG_WITH(random-seed,
 	[random_seed=$withval], [random_seed=yes])
 
 AC_ARG_WITH(keventd,
-        AS_HELP_STRING([--with-keventd], [Enable built-in keventd, default: no]),, [with_keventd=no])
+        AS_HELP_STRING([--with-keventd], [Enable built-in keventd device manager, default: yes]),, [with_keventd=yes])
 
 AC_ARG_WITH(sulogin,
         AS_HELP_STRING([--with-sulogin@<:@=USER@:>@], [Enable built-in sulogin, optional USER to request password for (default root), default: no.]),[sulogin=$withval],[with_sulogin=no])