Now that the former "linux" branch is being retired, change the portable sync script to reference "portable".

Don't pass '-v' to the command buitin.

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

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.

Ensure that the remote GH repo is fetched from.

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

Add detection for headers which are included portably, and remove them.

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