Refactor: libcrmcommon: g_string_append_printf -> pcmk__g_strcat()

g_string_*printf() is not strictly necessary when all the interpolated
objects are strings. A sequence of g_string_append() calls suffices and
performs much better (with -O2 compiler optimization enabled).
pcmk__g_strcat() is a convenience wrapper for such a sequence of
g_string_append() calls.

This change yields a significant performance boost on crm_simulate
tests.

Ref T152

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