Feature: libcrmcommon: Internal constant for number of Pacemaker RCs

Convert PCMKN_RC to const int pcmk__n_rc in lib/common/results.c,
and declare it as extern in results_internal.h. This way we can access
the number of negative Pacemaker return codes from anywhere internally.
We'll use this next to improve some loop logic in
pcmklist_result_codes().

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>