Implement "want" dependency

The want dependency is similar to the use dependency. If a service
script, for example called service1, adds "want service2" to its depend
function, OpenRC will attempt to start service2, if it exists on the
system,  when service1 is started.

However, service1 will start regardless of the status of
service2.

X-Gentoo-Bug: 406021
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=406021

sh/openrc-run.sh.in

@@ -66,6 +66,9 @@ need() {
 use() {
 	[ -n "$*" ] && echo "use $*"
 }
+want() {
+	[ -n "$*" ] && echo "want $*"
+}
 before() {
 	[ -n "$*" ] && echo "before $*"
 }