pidref: add helpers for managing PidRef on the heap

pidref: add helpers for managing PidRef on the heap

Usually we want to embed PidRef in other structures, but sometimes it
makes sense to allocate it on the heap in case it should be used
standalone. Add helpers for that.

Primary usecase: use as key in Hashmap objects, that for example map
process to unit objects in PID 1.

This adds pidref_free()/pidref_freep() for freeing such an allocated
struct, as well as pidref_dup() (for duplicating an existing PidRef
on the heap 1:1), and pidref_new_pid() (for allocating a new PidRef from a
PID).