@brief log facility
`--log.facility name`

If this option is set, then in addition to output being directed to the
standard output (or to a specified file, in the case that the command line
log.file option was set), log output is also sent to the system logging
facility. The *arg* is the system log facility to use. See syslog for
further details.

The value of *arg* depends on your syslog configuration. In general it
will be *user*. Fatal messages are mapped to *crit*, so if *arg*
is *user*, these messages will be logged as *user.crit*.  Error
messages are mapped to *err*.  Warnings are mapped to *warn*.  Info
messages are mapped to *notice*.  Debug messages are mapped to
*info*.  Trace messages are mapped to *debug*.