ps: Allow -q option multiple times, and separated by spaces or commas

Several differences here:
- Passing `-q` multiple times will add them together, instead of the
  last one overwriting the previous ones.
- `-q` PIDs can be separated by commas as well as spaces.
- We check that the PIDs are integers while parsing the arguments,
  instead of later on.

The "parse a list of things as an option" is extracted into a helper
function, because we're going to want the same logic for `-g`, `-G`,
`-p`, `-t`, `-u`, and `-U`.

Base/usr/share/man/man1/ps.md

@@ -18,7 +18,7 @@ For each process, print its PID (process ID), to which TTY it belongs, and invok
 * `-a`: Consider all processes that are associated with a TTY.
 * `-A` or `-e`: Consider all processes, not just those in the current TTY.
 * `-f`: Also print for each process: UID (as resolved username), PPID (parent PID), and STATE (Runnable, Sleeping, Selecting, Reading, etc.)
-* `-q pid-list`: Only consider the given PIDs, if they exist.
+* `-q pid-list`: Only consider the given PIDs, if they exist. `pid-list` is a list of PIDs, separated by commas or spaces.
 
 ## Examples