core: in --user mode, report READY=1 as soon as basic.target is reached (#7102)

When a user logs in, systemd-pam will wait for the user manager instance to
report readiness. We don't need to wait for all the jobs to finish, it
is enough if the basic startup is done and the user manager is responsive.

systemd --user will now send out a READY=1 notification when either of two
conditions becomes true:
- basic.target/start job is gone,
- the initial transaction is done.

Also fixes #2863.