Add dependency resolver for plugins.

This has been something I wanted to avoid, but then I stumbled so hard
on it myself I simply had to add it.  With this commit finit actually
comes of age -- it's now possible to do anything a modern init can --
only faster! :-)

Included in this commit is the OpenBSD version of sys/queue.h, which
unline all versions of the same file on Linux has the _FOREACH_SAFE
macros for iterating over a linked list.

The plugins are now in a TAILQ, loaded in alphabetic order and, unless
the dependency resolver says otherwise, added last in the queue.  So,
naming your plugins according to standard sysvinit rules: S01, S02,
etc. takes care of the most basic dependency resolution.  When that is
not sufficient, simply add a line ".depends = { "other_plugin",
"another_plugin" }," to your plugin_t definition.  Be careful though
for circular dependencies, we don't check for those.

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

Makefile

@@ -35,7 +35,7 @@ PKG         = $(NAME)-$(VERSION)
 DEV         = $(NAME)-dev
 ARCHIVE     = $(PKG).tar.xz
 EXEC        = finit reboot
-HEADERS     = plugin.h svc.h helpers.h
+HEADERS     = plugin.h svc.h helpers.h queue.h
 DISTFILES   = LICENSE README ChangeLog finit.conf services
 OBJS        = finit.o conf.o helpers.o sig.o svc.o plugin.o
 OBJS       += strlcpy.o