Follow-up to c9fe9afa, proper fix for HOOK_BASFFS_UP mess

Instead of the revert in c9fe9afa, we restore HOOK_BASFFS_UP to its
proper place at the end of fs_mount_all().  For this to not cause any
regressions we add a new hook, HOOK_SVC_PLUGIN, and update all plugins
that call service_register() to run at the new hook.

This will cause regressions for external 3rd party plugins that rely on
HOOK_BASEFS_UP to be called at its previous postion.  Nevertheless, this
is the proper fix to the problem.

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

src/plugin.h

@@ -72,6 +72,7 @@
 	CHOOSE(HOOK_MOUNT_ERROR,     "hook/mount/error"),	\
 	CHOOSE(HOOK_MOUNT_POST,      "hook/mount/post"),	\
 	CHOOSE(HOOK_BASEFS_UP,       "hook/mount/all"),		\
+	CHOOSE(HOOK_SVC_PLUGIN,      "hook/svc/plugin"),	\
 	CHOOSE(HOOK_NETWORK_UP,      "hook/net/up"),		\
 	CHOOSE(HOOK_SVC_UP,          "hook/svc/up"),		\
 	CHOOSE(HOOK_SYSTEM_UP,       "hook/sys/up"),		\