Remove support for service callback plugins

This patch removes the now heavily outdated (and non-working) support
for callback plugins.  I.e. plugins that attach themselves to services
listed in /etc/finit.conf and can be queried to determine if a service
should be started or stopped.

The Finit3 support for adding/removing services at runtime with plain
text *.conf files in /etc/finit.d and `initctl reload`, this old, and
rather cumbersome service callback feature, is not longer supported.

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

doc/plugins.md

@@ -1,10 +1,11 @@
-Hooks, Callbacks & Plugins
-==========================
+Hooks & Plugins
+===============
 
-Finit provides only the bare necessities for starting and supervising
-processes, with an emphasis on *bare* — for your convenience it does
-however come with support for hooks, service callbacks and plugins that
-can used to extend finit with.
+Finit can be extended to add general functionality in the form of I/O
+monitors, built-in inetd services, or hook plugins.
+
+The following sections detail existing plugins and hook points.  For
+more information, see the plugins listed below.
 
 
 Plugins
@@ -124,20 +125,6 @@ they are called I/O plugins and are called from the finit main loop when
 more help and ideas.
 
 
-Callbacks
----------
-
-Callback plugins are called by finit right before a process is started,
-or restarted if it exits.  The callback runs as a separate process and
-receives a pointer to the `svc_t` of the service, with all command line
-parameters free to modify as needed.
-
-All the callback needs to do is respond with one of: `SVC_STOP (0)`
-tells Finit to *not* start the service, `SVC_START (1)` to start the
-service, or `SVC_RELOAD (2)` to have finit signal the process with
-`SIGHUP`.
-
-
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