gcore: add -h|--help options, and improve help/usage message output

Like the previous commit, this copies a lot from:

  commit fb2ded33c1e519659743047ed7817166545b6d91
  Date:   Fri Dec 20 12:46:11 2024 -0800

      Add gstack script

And adds -h | --help options to the gcore script, and smartens up the
help and usage output messages.

The usage text is now split over several lines (as it was getting a
bit long), and an input error suggests using `--help` instead of
printing the full usage string.

These changes bring gcore and gstack closer in behaviour.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32325
Approved-By: Tom Tromey <tom@tromey.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>