gdb/python: have PendingFrame methods accept keyword arguments

Update the two gdb.PendingFrame methods gdb.PendingFrame.read_register
and gdb.PendingFrame.create_unwind_info to accept keyword arguments.

There's no huge benefit for making this change, both of these methods
only take a single argument, so it is (maybe) less likely that a user
will take advantage of the keyword arguments in these cases, but I
think it's nice to be consistent, and I don't see any particular draw
backs to making this change.

For PendingFrame.read_register I've changed the argument name from
'reg' to 'register' in the documentation and used 'register' as the
argument name in GDB.  My preference for APIs is to use full words
where possible, and given we didn't support named arguments before
this change should not break any existing code.

There should be no user visible changes (for existing code) after this
commit.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Reviewed-By: Tom Tromey <tom@tromey.com>