core: port unit_main_pid() + unit_control_pid() to PidRef and drop unit_kill_common()

core: port unit_main_pid() + unit_control_pid() to PidRef and drop unit_kill_common()

This ports over unit_main_pid() + unit_control_pid() to return PidRef*
pointers (which also means the underlying UnitVTable function pointers
are changed accordingly).

This then uses te functions to simplify the unit_kill() call, by
avoiding the kill() vtable indirection and instead just suing
unit_main_pid() and unit_control_pid() directly.