Issue #223: add type:forking service option, with updated docs

- Add `type:forking` service option to trigger guessing pidfile to
   watch for, instead of `pid:!foo` option, which is not intuitive.
   This option may likely also survive into the new file format :)
 - Update docs and add examples
 - Update start-stop-serv.sh test case with this new variant

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

man/finit.conf.5

@@ -194,7 +194,21 @@ services and are supported using the same syntax as forking
 .Cm sysv
 services, using the
 .Cm pid:!/path/to/pidfile.pid
-command modifier syntax.
+command modifier syntax.  There is an alternative syntax that may be
+more intuitive, where Finit can also guess the PID file based on the
+daemon's command name:
+.Bd -unfilled -offset indent
+service type:forking ntpd -- NTP daemon
+.Ed
+.Pp
+Here we let BusyBox
+.Nm ntpd
+daemonize itself.  Finit uses the basename of the binary to guess the
+PID file to watch for the PID:
+.Pa /var/run/ntpd.pid .
+If Finit guesses wrong, you have to submit the full
+.Cm pid:!/path/to/file.pid
+option to your service stanza.
 .Pp
 .Sy Example: in the case of
 .Cm ospfd