gdb: new 'maint print frame-id' command

gdb: new 'maint print frame-id' command

When debugging a certain class of GDB bug, I often end up wanting to
know what GDB thinks the frame-id is in a particular frame.  It's
not too hard to pull this from some debug output, but I thought it
might be nice if there was a maintenance command that could tell us.

This commit adds 'maint print frame-id' which prints the frame-id of
the currently selected frame.  You can also pass a frame level number
to find the frame-id for a specific frame.

There's a new test too.