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>

src/Makefile.am

@@ -30,7 +30,7 @@ getty_CFLAGS         = -W -Wall -Wextra -std=gnu99
 getty_CFLAGS        += $(lite_CFLAGS)
 getty_LDADD          = $(lite_LIBS)
 
-keventd_SOURCES      = keventd.c iwatch.c iwatch.h util.c util.h
+keventd_SOURCES      = keventd.c keventd.h uevent.c util.c util.h
 keventd_CFLAGS       = -W -Wall -Wextra -std=gnu99
 keventd_CFLAGS      += $(lite_CFLAGS)
 keventd_LDADD        = $(lite_LIBS)