Add rc.conf.d support

This makes it possible to override settings in rc.conf by adding a
directory @SYSCONFDIR@/rc.conf.d and putting files in this directory.
The files will be processed in lexical order, and the last setting in
these files will be used.

sh/openrc-run.sh.in

@@ -184,6 +184,12 @@ unset _conf_d
 
 # Load any system overrides
 sourcex -e "@SYSCONFDIR@/rc.conf"
+if [ -d "@SYSCONFDIR@/rc.conf.d" ]; then
+	for _f in "@SYSCONFDIR@"/rc.conf.d/*.conf; do
+		sourcex -e "$_f"
+	done
+fi
+
 
 # load service supervisor functions
 sourcex "@LIBEXECDIR@/sh/s6.sh"