Remove the concept of an instance. This means that a lot of functions have

Remove the concept of an instance. This means that a lot of functions have
been renamed. Accessors are prefixed with gdb-get, setters with gdb-set and
the word instance has been removed from many function and variable names.
(gdb-display-buffer): Protect source buffer as well
as GUD buffer when requesting new buffers.
(gdb-source-info): Two changes : Don't create display buffer
automatically, compute source window correctly.