Refactor: fencer: separate CIB-related code into own source file

No code changes, other than exposing some previously static symbols.
(Wrappers would help avoid that, but I didn't want to get that deep at this
point.)

Before:

1782 daemons/fenced/pacemaker-fenced.c
 318 daemons/fenced/pacemaker-fenced.h

After:

915 daemons/fenced/pacemaker-fenced.c
893 daemons/fenced/fenced_cib.c
328 daemons/fenced/pacemaker-fenced.h