Fix syslog logging on FreeBSD

The UNIX domain socket to use on FreeBSD is /var/run/log.
See syslogd FreeBSD man page: https://www.freebsd.org/cgi/man.cgi?query=syslogd&sektion=8

borgmatic/logger.py

@@ -151,6 +151,8 @@ def configure_logging(
             syslog_path = '/dev/log'
         elif os.path.exists('/var/run/syslog'):
             syslog_path = '/var/run/syslog'
+        elif os.path.exists('/var/run/log'):
+            syslog_path = '/var/run/log'
 
     if syslog_path and not interactive_console():
         syslog_handler = logging.handlers.SysLogHandler(address=syslog_path)