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>

doc/service.md

@@ -27,7 +27,17 @@ Finit can *not* start and monitor a daemon that:
 
 ### Forks to Background w/ PID File
 
-    service pid:!/run/serv.pid serv       -- Forking service
+There are two syntax variants, type 1 and type 2.  The former is the
+traditional one used also for `sysv` start/stop scripts, and the latter
+is inspired by systemd, with a twist -- it lets Finit guess the pifdile
+to look for based on the standard path and the basename of the command.
+
+    service pid:!/run/serv.pid serv       -- Forking service, type 1
+    service type:forking       serv       -- Forking service, type 2
+
+In this example the resulting files to watch for are `/run/serv.pid` and
+`/var/run/serv.pid`, respectively.  On most modern Linux systems this is
+the same directory (`/var/run` is a symlink to `../run`).
 
 ### Runs in Foreground w/ PID File