core: optionally create LOGIN_PROCESS or USER_PROCESS utmp entries

core: optionally create LOGIN_PROCESS or USER_PROCESS utmp entries

When generating utmp/wtmp entries, optionally add both LOGIN_PROCESS and
INIT_PROCESS entries or even all three of LOGIN_PROCESS, INIT_PROCESS
and USER_PROCESS entries, instead of just a single INIT_PROCESS entry.

With this change systemd may be used to not only invoke a getty directly
in a SysV-compliant way but alternatively also a login(1) implementation
or even forego getty and login entirely, and invoke arbitrary shells in
a way that they appear in who(1) or w(1).

This is preparation for a later commit that adds a "machinectl shell"
operation to invoke a shell in a container, in a way that is compatible
with who(1) and w(1).