Refactor: pacemakerd: Use as_root boolean in pcmk_child_t

There are really only two options: root and CRM_DAEMON_USER. The
generality of pcmk_child_t:uid made things a bit harder to follow. Plus
the name "uid" suggested a numeric UID.

Using an as_root boolean value simplifies the code a bit and allows us
to use pcmk_daemon_user() instead of crm_user_lookup(). This way, we
only have to do the lookup once. After that, the uid and gid are cached
in static variables within pcmk_daemon_user().

There are probably other opportunities to take advantage of the fact
that uid and gid are either root or the CRM_DAEMON_USER's uid and gid.

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