add customisable variable to control source buffer locking

this should really just be the single commit, 'add customisable variable to control source buffer locking' but was based on 'https://github.com/rocky/emacs-dbgr/pull/23' (the other two commits) so to avoid giving you merge conflicts given the code overlaps

the reason behind the three commits was the feeling i had of needing to 'regain control of the debugging process / buffers'. the default setup felt restrictive. i like editing on the fly (and thus restarting the process as and when the edits require it), and i like scrolling around the source buffer

the default short-key mode map also overrode many of my customisations, and try as i might, i failed to change to the map in any of the standard emacsy ways i knew, possibly due to the mix of populating the map on the fly, and the setup calls being initiated from the debugger process' buffer

this could all be unnecessary and i may have missed an intended trick, however, supporting standard emacs features like hooks is certainly preferable for me, so hence the hope of a merge

thanks for all your work on the project