API: libcrmcommon: Deprecate pcmk_daemon_user()

We don't need to offer a way to get a user's UID and GID as part of the
public API.

CRM_DAEMON_USER is available via crm_config.h. External callers can use
standard system utilities like getpwnam() to get the UID and GID if
necessary.

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