keventd: load udev rules in udev's order, with masking

The rules directories were read one after the other, each sorted on
its own.  udev instead sorts all files from all directories together
and lets a file mask one of the same name in a lower priority
directory.  The difference broke both standard override idioms
without a sound: copying a rules file to /etc/udev/rules.d/ to edit
it left the original running too, and symlinking a name to /dev/null
disabled nothing.

Collect everything with glob(), drop entries shadowed by a later
directory, and sort the survivors by filename, same as udev and the
tmpfiles.d handling in Finit itself.  The -r directory sits above
/etc/ so a test ruleset can override anything shipped.

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

keventd/keventd.c

@@ -660,7 +660,7 @@ static int usage(int rc)
 		"  -h        Show this help text\n"
 		"  -n        Run in foreground (no daemon)\n"
 		"  -p        Passive mode: power supply events only (no device management)\n"
-		"  -r DIR    Extra rules directory (in addition to standard udev paths)\n"
+		"  -r DIR    Extra rules directory, overrides the standard udev paths\n"
 		"  -S        Settle: wait until kernel uevent queue is quiet, then exit\n"
 		"  -t SEC    Settle timeout in seconds (default: 30, with -S)\n"
 		"  -v        Show version\n"