gdb: make use add_setshow_prefix_cmd in gnu-nat.c

In gnu-nat.c we currently implement some set/show prefix commands
"manually", that is, we call add_prefix_cmd, and assign a set and show
function to each prefix command.

These set/show functions print an error indicating that the user
didn't type a complete command.

If we instead switch to using add_setshow_prefix_cmd then we can
delete the set/show functions, GDB provides some default functions,
which give a nice help style summary that lists all of the available
sub-commands, along with a one line summary of what each does.

Though this clearly changes the existing behaviour, I think this
change is acceptable as the new behaviour is more inline with other
set/show prefix commands, and the new behaviour is more informative.

This change will conflict with Tom's change here:

  https://sourceware.org/pipermail/gdb-patches/2022-January/184724.html

Where Tom changes the set/show functions that I delete.  My suggestion
is that the set/show functions still be deleted even after Tom's
patch (or instead of Tom's patch).

For testing I've build GDB on GNU/Hurd, and manually tested these
functions.  I did a grep over the testsuite, and don't believe the
existing error messages are being checked for in any tests.