Low: libcrmcommon: use return code correctly when getting daemon user ID

Previously, pcmk_daemon_user() would return pcmk_ok only on the first
successful call -- subsequent (successful) calls would always return
pcmk_err_generic, even though the IDs were available.

This didn't cause problems because all the callers except one checked whether
the return value was negative, and pcmk_err_generic isn't negative, so they
treated it like success. Now, pcmk_daemon_user() will always return a negative
value for errors, so everything now works as intended rather than by accident.

The one caller that checked for pcmk_ok was crm_add_logfile(). The effect would
be that if crm_add_logfile() were called after pcmk_daemon_user() had already
been called at least once, it would not change the ownership and mode of the
log file. This will now work correctly.