keventd: one condition emitter, one file reader, one clock

Three copies of read-first-line-from-a-file (sysfs_read_file, a
static sysfs_read, fgetline), two condition emitters symlinking the
same reconf marker (cond_emit, sys_cond), two seqnum-stable loops,
and five hand-rolled logit() prototypes, none with a printf
attribute, so no format string was ever checked.

Keep one of each: cond_emit() is exported and mkpaths its parents,
which also retires init_dev_condition_dir() and the sys/ directory
priming loop; logit() moves to keventd.h with the format attribute;
the stability test becomes seqnum_stable(), shared by the coldplug
gate and settle; kev_now_ms() serves dbus.c too.  rule_ctx_free()
and kev_seq_baseline() had one caller each in their own files, now
static.

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

keventd/rules.c

@@ -51,9 +51,6 @@
 #include "sysfs.h"
 #include "udevdb.h"
 
-/* Forward declaration from keventd.c */
-void logit(int prio, const char *fmt, ...);
-
 /* ----- key-name lookup -------------------------------------------------- */
 
 static const struct {