Add gdb.current_language and gdb.Frame.language

Add gdb.current_language and gdb.Frame.language

This adds the gdb.current_language function, which can be used to find
the current language without (1) ever having the value "auto" or (2)
having to parse the output of "show language".

It also adds the gdb.Frame.language, which can be used to find the
language of a given frame.  This is normally preferable if one has a
Frame object handy.