shortkey, clean up 'realgud-short-key-mode-setup' (whitespace only)

-give nesting a chance by removing mix of tabs and spaces
shortkey, add mode hook call at setup

-enable 'realgud-short-key-mode-hook' for those who might need it
add customisable variable to control source buffer locking

-allow users to control whether the source buffers associated with
the debugger process(es) are locked (read-only mode enabled) when
debugging is in progress