core: reference main/control pid of .service units via PidRef

core: reference main/control pid of .service units via PidRef

The first conversion to PidRef. It's mostly an excercise of
search/replace, but with some special care taken for life-cycle (i.e. we
need to destroy the structure properly once done, to release the pidfd).

It also uses pidfd based killing for some of the killing but leaves most
as it is to make the conversion minimal.