portable: maintscripts: fix command use Don't pass '-v' to the command buitin.

portable: maintscript: wrap nproc Wrap nproc for non-Linux systems so this script can run on, say, FreeBSD.

portable: maintscript: use env for bash Set this script's shebang line to use env to look for bash, as bash isn't always guaranteed to be installed to /bin, especally on BSD systems.

portable: maintscript: add GH remote Ensure that the remote GH repo is fetched from.

portable: maintscripts: reverse cherrypicks When cherry-picking commits from main, do so in reverse order so they match more linearly with main.

portable: maintscript: add header detection Add detection for headers which are included portably, and remove them.

portable: add maintscripts This introduces an area for helper scripts when dealing with -portable syncing from upstream, releases, etc. See the README.md file for further information.