This option allows the user to change which colums are displayed
by giving comma or space separated list of column format specifiers.
A column format specifier is of the form: `COLUMN_NAME[=COLUMN_TITLE]`.
Where `COLUMN_NAME` is any of: uid, pid, ppid, pgid, sid, state, tty,
or cmd. Specifying a `COLUMN_TITLE` will change the name shown in the
column header.
`COLUMN_TITLE` may be blank. If all given column titles
are blank, the header is omitted.
Previously, it was assumed that only one filtering option, such as
`-u` or `-p` would be used at a time. With this PR, processes are now
shown if they match any of the specified filters.
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`.
I can't write these manpages ad-hoc, and in most cases I don't want to
remove the link because it is justified. The hope is that with this
FIXME in place, there is more motivation to write these manpages for
someone who knows enough about them. Or at least we will introduce fewer
dead links in the future, making Help more useful.
