units: introduce remote-fs-setup.target to pull in dependencies from remote mounts

This introduces remote-fs-setup.target independently of
remote-fs-pre.target. The former is only for pulling things in, the
latter only for ordering.

The new semantics:

remote-fs-setup.target: is pulled in automatically by all remote mounts.
Shall be used to pull in other units that want to run when at least one
remote mount is set up. Is not ordered against the actual mount units,
in order to allow activation of its dependencies even 'a posteriori',
i.e. when a mount is established outside of systemd and is only picked
up by it.

remote-fs-pre.target: needs to be pulled in automatically by the
implementing service, is otherwise not part of the initial transaction.
This is ordered before all remote mount units.

A service that wants to be pulled in and run before all remote mounts
should hence have:

a) WantedBy=remote-fs-setup.target -- so that it is pulled in

b) Wants=remote-fs-pre.target + Before=remote-fs-pre.target -- so that
   it is ordered before the mount point, normally.

src/core/mount.c

@@ -441,7 +441,7 @@ static int mount_add_quota_links(Mount *m) {
 static int mount_add_default_dependencies(Mount *m) {
         int r;
         MountParameters *p;
-        const char *after;
+        const char *after, *setup;
 
         assert(m);
 
@@ -456,15 +456,24 @@ static int mount_add_default_dependencies(Mount *m) {
         if (path_equal(m->where, "/"))
                 return 0;
 
-        if (mount_is_network(p))
+        if (mount_is_network(p)) {
                 after = SPECIAL_REMOTE_FS_PRE_TARGET;
-        else
+                setup = SPECIAL_REMOTE_FS_SETUP_TARGET;
+        } else {
                 after = SPECIAL_LOCAL_FS_PRE_TARGET;
+                setup = NULL;
+        }
 
-        r = unit_add_two_dependencies_by_name(UNIT(m), UNIT_WANTS, UNIT_AFTER, after, NULL, true);
+        r = unit_add_dependency_by_name(UNIT(m), UNIT_AFTER, after, NULL, true);
         if (r < 0)
                 return r;
 
+        if (setup) {
+                r = unit_add_dependency_by_name(UNIT(m), UNIT_WANTS, setup, NULL, true);
+                if (r < 0)
+                        return r;
+        }
+
         r = unit_add_two_dependencies_by_name(UNIT(m), UNIT_BEFORE, UNIT_CONFLICTS, SPECIAL_UMOUNT_TARGET, NULL, true);
         if (r < 0)
                 return r;

src/core/special.h

@@ -54,6 +54,7 @@
 #define SPECIAL_INITRD_ROOT_FS_TARGET "initrd-root-fs.target"
 #define SPECIAL_REMOTE_FS_TARGET "remote-fs.target"       /* LSB's $remote_fs */
 #define SPECIAL_REMOTE_FS_PRE_TARGET "remote-fs-pre.target"
+#define SPECIAL_REMOTE_FS_SETUP_TARGET "remote-fs-setup.target"
 #define SPECIAL_SWAP_TARGET "swap.target"
 #define SPECIAL_BASIC_TARGET "basic.target"