keventd: convert main loop to libuev

Replace the raw poll() + global running/reload_rules flag dance with
libuev.  All four signals (SIGUSR1, SIGTERM, SIGHUP, SIGCHLD) are now
handled via uev_signal_init() -- libuev uses signalfd internally so the
handlers run in normal main-loop context, not async signal context.

The SIGCHLD handler waitpid(-1, WNOHANG) reaps any subprocess (modprobe
loads, rule RUN+= helpers) instead of the old SIG_IGN auto-reap trick.
Synchronous waitpid() in modprobe_load() and run_program() still works
because signalfd queues the signal -- it only reaches sigchld_cb on the
next main-loop iteration.

The netlink receive loop becomes uevent_cb() registered via uev_io_init(),
with the receive buffer passed via the watcher's arg slot (avoids
file-static state).  ENOBUFS and EINTR/EAGAIN are still tolerated, any
other recv() error still panic()s.

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

keventd/Makefile.am

@@ -15,8 +15,10 @@ keventd_CFLAGS     = -W -Wall -Wextra -std=gnu99
 # handled via explicit checks where present.  Matches systemd-udev policy.
 keventd_CFLAGS    += -Wno-format-truncation
 keventd_CFLAGS    += $(lite_CFLAGS)
+keventd_CFLAGS    += $(uev_CFLAGS)
 keventd_CFLAGS    += $(blkid_CFLAGS)
 keventd_LDADD      = $(lite_LIBS)
+keventd_LDADD     += $(uev_LIBS)
 keventd_LDADD     += $(blkid_LIBS)
 
 # Curated udev rules derived from eudev 3.2.14 (GPL-2.0-or-later).