[Ported from mercurial] JOE prompts the user to create the backup dir if it doesn't exist + minor fix to chddir

[jjjordan]

This is a port of merge request 1 on sourceforge.

User is prompted with y/n on startup to create the backup dir if backup dir is set in the options. Chddir now is a function instead of a macro because chdir has difficulties with going to home (~) directories.

[jjjordan]

I've modified the original merge request to: merge latest, reformat to conventions, and reuse canonical to fix chddir behavior.

The open questions, as I see them are:

1. Do we want this prompt when the editor is opened?
2. Are we OK with making this specific change to chddir?

EDIT: mkpath needs some work still, since it won't handle ~user properly, but the code there is a little more complicated.
EDIT 2: This has been addressed but relies on the new chddir behavior, which I'm not sure about now. Should consider moving canonical into path.c
EDIT 3: I think prompting to create the backup directory the first time JOE tries to create a backup file would be more consistent with how JOE handles other situations. Moreover, there are already prompts during file save and rarely prompts at startup.