Support more POSIX/SUSv3 options:

• Change -p' to allow a list of process IDs, and -t' to allow a list of terminal names, instead of only a single value for each.
• Add the -A' option of SUSv3, which is exactly the same as -ax'.
• Add the `-G gidlist' (group id).
• Allow any of these "selector options" to be specified multiple times, and have `ps' keep adding to a given list -- instead of replacing the previously-specified values.
• Fix interactions between selector-options, so that: "If any are specified, ... ps shall select the processes represented by the inclusive OR of all the selection-criteria options." (from SUSv3)
• Add a -X' option, which is the reverse of the -x' option.

• various minor improvements in parsing and error handling.

This does not get us to match POSIX/SUSv3, but it gets us closer. The
-g pgidlist', -R ruserlist' and `-s sidlist' options mentioned in
freebsd-standards are still under debate, so they skipped for now.
It should be true that this introduces no user-visible incompatible
changes, except to support "new stuff" that was not supported before.