cron: New API for absolute timers

This patch adds support for at/cron-like jobs that need an absolute
timer.  The API supports waking up with an UEV_HUP callback event if the
wall clock changed due to NTP or user input while waiting.

- uev_cron_init()
- uev_cron_set()
- uev_cron_start()
- uev_cron_stop()

A very simple cronrun example/test program is also included.

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

examples/Makefile.am

@@ -1,4 +1,4 @@
-noinst_PROGRAMS = signal test joystick
+noinst_PROGRAMS = signal test joystick cronrun
 AM_CPPFLAGS     = -I$(top_srcdir)
 AM_LDFLAGS      = -L$(top_srcdir)
 LIBS            = $(LTLIBINTL)