core: accept "|" ExecStart= prefix to spawn target user's shell

When switching to another user it's oftentimes desirable to also spawn
the target user's shell. sudo supports this via -i flag, run0 currently
doesn't. We don't want to proactively query NSS ourselves, since
that would fall short when operating remotely. Let's instead teach
the service manager to spawn the command using the user's default shell.

I opted for "|" instead of "." in the end because the latter seems
a bit obscure. But happy to change it to something else if a better option
comes up.

TODO

@@ -720,10 +720,6 @@ Features:
 * machined: optionally track nspawn unix-export/ runtime for each machined, and
   then update systemd-ssh-proxy so that it can connect to that.
 
-* add a new ExecStart= flag that inserts the configured user's shell as first
-  word in the command line. (maybe use character '.'). Usecase: tool such as
-  run0 can use that to spawn the target user's default shell.
-
 * introduce mntid_t, and make it 64bit, as apparently the kernel switched to
   64bit mount ids