Accept CVS2SVN_INST directory from environment

Probably needs to be passed in from the crontab or whatever
cvs2git.options copied from cvs2git-example.options
Name cvs2git.options to look like an example.

Still don't want to impose cvs2git.options on other people, but
 branch master needs it
  because of the os.environ['REPOCVS'] hack
   because cvs2git won't set_project alongside --options
Use cvs2git.options file instead of command line args.

Remainder cherry-pick from f6ab24d0
Ensure canonified REPOCVS path gets to cvs2git.
Merge branch 'dev/cvs2svn240'

* dev/cvs2svn240:
  Ensure canonified REPOCVS path gets to cvs2git.
  Use cvs2git.options file instead of command line args.
  Name cvs2git.options to look like an example.
  cvs2git.options copied from cvs2git-example.options

Catches up with a00ec466

(I wanted not to ship cvs2git.options
 but there is no avoiding it for 2.4.0, forgotten exactly why)