core/service: rework ExecReload= + Type=notify-reload interaction, add ExecReloadPost=

core/service: rework ExecReload= + Type=notify-reload interaction, add ExecReloadPost=

When Type=notify-reload got introduced, it wasn't intended to be
mutually exclusive with ExecReload=. However, currently ExecReload=
is immediately forked off after the service main process is signaled,
leaving states in between essentially undefined. Given so broken
it is I doubt any sane user is using this setup, hence I took a stab
to rework everything:

1.  Extensions are refreshed (unchanged)
2.  ExecReload= is forked off without signaling the process
3a. If RELOADING=1 is sent during the ExecReload= invocation,
    we'd refrain from signaling the process again, instead
    just transition to SERVICE_RELOAD_NOTIFY directly and
    wait for READY=1
3b. If not, signal the process after ExecReload= finishes
    (from now on the same as Type=notify-reload w/o ExecReload=)
4.  To accomodate the use case of performing post-reload tasks,
    ExecReloadPost= is introduced which executes after READY=1

The new model greatly simplifies things, as no control processes
will be around in SERVICE_RELOAD_SIGNAL and SERVICE_RELOAD_NOTIFY
states.

See also: https://github.com/systemd/systemd/issues/37515#issuecomment-2891229652