Use generator to enable ostree-remount.service and ostree-finalize-staged.path

We struggled for a long time with enablement of our "internal units",
trying to follow the philosophy that units should only be enabled
by explicit preset.

See https://bugzilla.redhat.com/show_bug.cgi?id=1451458
and https://github.com/coreos/rpm-ostree/pull/1482
etc.

And I just saw chat (RH internal on a proprietary system sadly) where
someone hit `ostree-remount.service` not being enabled in CentOS8.

Thinking about this more, I realized we've shipped a systemd generator
for a long time and while its only role until now was to generate `var.mount`,
but by using it to force on our internal units, we don't require
people to deal with presets anymore.

Basically we're inverting things so that "if ostree= is on the kernel
cmdline, then enable our units" and not "enable our units, but have
them use ConditionKernelCmdline=ostree to skip".

Drop the weird gyrations we were doing around `ostree-finalize-staged.path`
too; forking `systemctl start` is just asking for bugs.

So after this, hopefully we won't ever again have to think about
distribution presets and our units.

src/libostree/ostree-sysroot-private.h

@@ -37,8 +37,7 @@ typedef enum {
   OSTREE_SYSROOT_DEBUG_TEST_FIFREEZE = 1 << 2,
   /* This is a temporary flag until we fully drop the explicit `systemctl start
    * ostree-finalize-staged.service` so that tests can exercise the new path unit. */
-  OSTREE_SYSROOT_DEBUG_TEST_STAGED_PATH = 1 << 3,
-  OSTREE_SYSROOT_DEBUG_TEST_NO_DTB = 1 << 4, /* https://github.com/ostreedev/ostree/issues/2154 */
+  OSTREE_SYSROOT_DEBUG_TEST_NO_DTB = 1 << 3, /* https://github.com/ostreedev/ostree/issues/2154 */
 } OstreeSysrootDebugFlags;
 
 typedef enum {