service: add ability to pin fd store

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.