Refactor: libcrmcommon: Drop pcmk__starts_with()

Use g_str_has_prefix() instead. Note, however, that the GLib function
logs an error if either of its arguments is NULL. So we NULL-check the
argument anywhere that we're not already assuming it's non-NULL (like
printing its value in a log message).

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