vim-genpatch: simplify global code dir variable usage

Basically override VIMCODE in the calling env with the path where you
want to put the vim source code repo.
vimball: remove unnecessary extractdir function
vimball: we don't need to use munged "private" functions names
vimball: convert filelines to int on initialization
vimball: drop duplicate parsing code for old header regex fallback
vimball: pep8 style cleanups