pid1: parse CPUAffinity= in incremental fashion

pid1: parse CPUAffinity= in incremental fashion

This makes the handling of this option match what we do in unit files. I think
consistency is important here. (As it happens, it is the only option in
system.conf that is "non-atomic", i.e. where there's a list of things which can
be split over multiple assignments. All other options are single-valued, so
there's no issue of how to handle multiple assignments.)

(cherry picked from commit 61fbbac1d517a0b3498a689c736c6ca918497904)

Related: #1734787