Major change, rename <svc/...> conditions to <pid/...> conditions

The <svc/foo> condition was created to synchronize starting services,
hence the abbreviation.  Example: the Quagga ripd needs to start after
the zebra daemon to ensure its UNIX domain socket is active, otherwise
events may be lost.

However, considering that synchronization was implemented with UNIX PID
files, e.g. waiting for /var/run/quagga/zebra.pid to be created, the
condition abbreviation name <svc/foo> was hard to understand by most
newcomers to Finit.  To make matters worse, a new feature to track or
even create PID files for services that don't create one themselves,
using the syntax 'pid:/path/to/foo.pid' was added.

Connecting the dots between these wasn't obvious.

This patch renames service conditions pid conditions and also adds
a compatibility wrapper to the Finit .conf parser.  Any condition
given in old .conf files with 'svc/' prefix are internally renamed
'pid/', along with a LOG_INFO notice in syslog.

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

README.md

@@ -140,7 +140,7 @@ Here is another example where we instruct Finit to not start BusyBox
 `ntpd` until `syslogd` has started properly.  Finit waits for `syslogd`
 to create its PID file, by default `/var/run/syslogd.pid`.
 
-    service [2345] log <svc/sbin/syslogd> /usr/sbin/ntpd -n -N -p pool.ntp.org
+    service [2345] log <pid/sbin/syslogd> /usr/sbin/ntpd -n -N -p pool.ntp.org
     service [S12345] /sbin/syslogd -n -- Syslog daemon
 
 Notice the `log` keyword, BusyBox `ntpd` uses `stderr` for logging when
@@ -150,7 +150,7 @@ to the system log daemon using the command line `logger(1)` tool.
 A service, or task, can have multiple dependencies listed.  Here we wait
 for *both* `syslogd` to have started and basic networking to be up:
 
-    service [2345] log <svc/sbin/syslogd,net/route/default> /usr/sbin/ntpd -n -N -p pool.ntp.org
+    service [2345] log <pid/sbin/syslogd,net/route/default> /usr/sbin/ntpd -n -N -p pool.ntp.org
 
 If either condition fails, e.g. loss of networking, `ntpd` is stopped
 and as soon as it comes back up again `ntpd` is restarted automatically.