Add 'notify:pid' style readiness notifaction and 'readiness none'

This change expands the readiness notification system in Finit with the
native 'pid' style, which will remain the default readiness in Finit 4.x

For systems that want to transition to Finit 5.x early, a global option
to set 'readiness none' in /etc/finit.conf, has been added.  This change
the service default notification mode to 'notify:none', which can also
be set by Finit 4.x ('readiness pid') for select services.

Fixes #386.

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

doc/config.md

@@ -356,17 +356,31 @@ most notably systemd and s6, other methods for signaling "readiness"
 arrived and daemons were adapted to these new schemes to a larger
 extent.
 
-As of Finit v4.4 partial support for systemd and s6 is available.  The
-native PID file mode of operation is still default, the other's can be
-defined per service, like this:
+As of Finit v4.4 partial support for systemd and s6 style readiness
+notification is available, and the native PID file mode of operation is,
+as of Finit v4.6 optional, by default it is still enabled, but this can
+be changed in `finit.conf`:
 
+    readiness none
+
+This will be made the default in Finit 5.0.  In this mode of operation,
+every service needs to explicitly declare their readiness notification,
+like this:
+
+    service notify:pid     watchdogd
     service notify:systemd foo
     service notify:s6      bar
+    service notify:none    qux
+
+The `notify:none` syntax is for completeness in systems which run in
+`readiness pid` mode (default).  Services declared with `notify:none`
+will assert ready condition as soon as Finit has started them, e.g.,
+`service/qux/ready`.
 
 To synchronize two services the following condition can be used:
 
-    service notify:systemd    A
-    service <service/A/ready> B
+    service notify:pid                 watchdogd
+    service <service/watchdogd/ready>  stress-ng --cpu 8
 
 For details on the syntax and options, see below.