Refactor: get rid of pcmk_handler as a global variable

It is still more covenient to split definition and actual use across two
files (pacemaker.c and main.c, respectively), so we at least make the
semi-hidden nature (no presence in any of the actual header files)
apparent with the double-underscore after initial namespace prefix
convenience.

Also fix a latent "no extern storage class specifier specified" in case
of compiling with -fno-common/default since GCC 10 (for missing "extern"
qualifier).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>