Fix #95: Create PID file if program does not

This patch adds support for creating, and removing, a PID file for
programs (services) that for some reason does not create one.  With
Finit this is very useful since it allows us to use our condition
support even for "broken" daemons.

To enable this for a service, use the following syntax:

    service pid[:[/path/to/]file[.pid]] /path/to/daemon

I.e., `pid` is a required argument if you want to enable the feature,
but the rest is optional:

    pid             --> /run/`basename $CMD`.pid
    pid:file        --> /run/file.pid
    pid:file.dip    --> /run/file.dip
    pid:/mnt/k      --> /mnt/k.pid
    pid:/mnt/k.foo  --> /mnt/k.foo

There is an additional feature with this patch.  For daemons that run
with a different identiy one usually wants to create the PID file with
that identity, e.g.:

    service inadyn -I dyn     -- Dyn.com DDNS client
    service inadyn -I freedns -- FreeDNS DDNS client

Here inadyn will look for /etc/dyn.conf and create /run/dyn.pid, and
/etc/freedns.conf and create /run/freedns.pid, respetively.  While Finit
will in both cases assume the PID file is /run/inadyn.pid ... err fail!

To get out of this mess the `pid` argument can be used to tell Finit
what the real PID file the daemon creates is called:

    service pid:!dyn     inadyn -I dyn     -- Dyn.com DDNS client
    service pid:!freedns inadyn -I freedns -- FreeDNS DDNS client

The same composition rules listed above applies also to the `!` variant.

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

src/svc.h

@@ -90,6 +90,7 @@ typedef struct svc {
 
 	/* Service details */
 	pid_t	       pid;
+	char           pidfile[MAX_ARG_LEN];
 	long           start_time;     /* Start time, as seconds since boot, from sysinfo() */
 	const svc_state_t state;       /* Paused, Reloading, Restart, Running, ... */
 	svc_type_t     type;	       /* Service, run, task, inetd, ... */