man/systemd.service: advise Type=exec instead of Type=simple

man/systemd.service: advise Type=exec instead of Type=simple

The descriptions of various options are reworked: first say what protocol
actually is, i.e. describe what type of notification the manager waits
for. Only after that describe various steps and things the service should
do. Also, apply some paragraph breaks.

Instead of recommending Type=simple, recommend Type=exec. Say explicitly that
Type=simple, Type=forking are not recommended. Type=simple ignores failure in a
way that doesn't make any sense except as a historical accident. We introduced
'exec' instead of changing 'simple' to keep backwards-compatiblity, but
'simple' is not very useful. 'forking' works, but is inefficient: correctly
programming the interface requires a lot of work, and at runtime, the
additional one or two forks are just a waste of CPU resources. Furthermore, we
now understand that because of COW traps, they may also increase memory
requirements. There is really no reason to use 'forking', except if it's
already implemented and the code cannot be changed to use 'notify'.

Also, remove the recommendations to use Type=simple to avoid delaying boot. In
most cases, if the service can support notifications about startup, those
should be done.

Overall, for new services, "notify", "notify-reload", and "dbus" are the
types that make sense.