service: add ability to pin fd store

Oftentimes it is useful to allow the per-service fd store to survive
longer than for a restart. This is useful in various scenarios:

1. An fd to some security relevant object needs to be stashed somewhere,
   that should not be cleaned automatically, because the security
   enforcement would be dropped then.

2. A user namespace fd should be allocated on first invocation and be
   kept around until the user logs out (i.e. systemd --user ends), á la
   #16328 (This does not implement what #16318 asks for, but should
   solve the use-case discussed there.)

3. There's interest in allow a concept of "userspace reboots" where the
   kernel stays running, and userspace is swapped out (i.e. all services
   exit, and the rootfs transitioned into a new version of it) while
   keeping some select resources pinned, very similar to how we
   implement a switch root. Thus it is useful to allow services to exit,
   while leaving their fds around till the very end.

This is exposed through a new FileDescriptorStorePreserve= setting that
is closely modelled after RuntimeDirectoryPreserve= (in fact it reused
the same internal type), since we want similar behaviour in the end, and
quite often they probably want to be used together.

src/basic/unit-def.c

@@ -201,6 +201,7 @@ static const char* const service_state_table[_SERVICE_STATE_MAX] = {
         [SERVICE_FAILED]                     = "failed",
         [SERVICE_DEAD_BEFORE_AUTO_RESTART]   = "dead-before-auto-restart",
         [SERVICE_FAILED_BEFORE_AUTO_RESTART] = "failed-before-auto-restart",
+        [SERVICE_DEAD_RESOURCES_PINNED]      = "dead-resources-pinned",
         [SERVICE_AUTO_RESTART]               = "auto-restart",
         [SERVICE_CLEANING]                   = "cleaning",
 };

src/basic/unit-def.h

@@ -146,6 +146,7 @@ typedef enum ServiceState {
         SERVICE_FAILED,
         SERVICE_DEAD_BEFORE_AUTO_RESTART,
         SERVICE_FAILED_BEFORE_AUTO_RESTART,
+        SERVICE_DEAD_RESOURCES_PINNED,  /* Like SERVICE_DEAD, but with pinned resources */
         SERVICE_AUTO_RESTART,
         SERVICE_CLEANING,
         _SERVICE_STATE_MAX,