Follow-up to f4a0b99: track modifications of service env: files

Add support for monitoring environment files for services declared with
the `env:[-]/path/to/file` option.  The default path to such files has
been chosen to follow Debian and Buildroot /etc/default/*

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

doc/config.md

@@ -68,6 +68,34 @@ document [Finit Services](service.md).
 > examples in the `contrib/` directory, which can be used as a base.
 
 
+Service Environment
+-------------------
+
+Finit supports sourcing environment variables from `/etc/default/*`, or
+similar `--with-sysconfig=DIR`.  This is a common pattern from SysV init
+scripts, where the start-stop script is a generic script for the given
+service, `foo`, and the options for the service are sourced from the
+file `/etc/default/foo`.  Like this:
+
+* `/etc/default/foo`:
+
+        FOO_OPTIONS=--extra-arg="bar" -s -x
+
+* `/etc/finit.conf`:
+
+        service [2345] env:-/etc/default/foo foo -n $FOO_OPTIONS -- Example foo daemon
+
+Here the service `foo` is started with `-n`, to make sure it runs in the
+foreground, and the with the options found in the environment file.  With
+the `ps` command we can see that the process is started with:
+
+    foo -n --extra-arg=bar -s -x
+
+> **Note:** the leading `-` determines if Finit should treat a missing
+> environment file as blocking the start of the service or not.  When
+> `-` is used, a missing environment file does *not* block the start.
+
+
 Service Wrapper Scripts
 -----------------------