yeah, yet another patch ... :-]

yeah, yet another patch ... :-]
- fix config file name in $HOME; This didn't match the documentation.
  (doesn't cost us anything to check ~/.fg-submit first, and then ~/.fg-submitrc)
- don't use mktemp for the backup files. Some outdated distributions
  (Debian) come with a version that mandates six X, which is just too ugly.
  Just find the first free slot with sequential number. That isn't thread
  safe, but mktemp isn't either, so ...  (Should be using "lockfile", but
  its availability on CygWin is questionable. And it's not *that* important.)
- some more documentation
- some cleanup, too, of course