MFC r258894: Make rc(8) re-source rc.conf upon receipt of SIGALRM.

The rc system aggressively caches the contents of /etc/rc.conf in order to
improve boot performance; this produces arguably astonishing (non-)results
if /etc/rc.conf is modified during the boot process.  This commit provides
a mechanism for explicitly requesting that rc.conf be reloaded.

Approved by:	re (rodrigc)