By using --cores argument to rootfs.py, JOBS= is set in the
live-bootstrap environment, and -j${JOBS} is used on builds. This speeds
larger packages up significantly.
A fair number of packages do not build properly with parallelism. Most
of these, at least for now, are disabled with -j1.
When not running as root, the shebang workaround requires write
permissions to modify help2man. Since the original file is read-only,
permission must be granted explicitly.
Also fix the double '/' in the /usr prefix while at it.