Bourne shell -> POSIX shell

[ziaee]

Summary

The FreeBSD shell is a POSIX compatible shell. It evolved over several
decades from the Almquist shell, which was preceeded a decade before
that by the Bourne shell. Most readers today have never seen a Bourne
shell. If someone wants to learn to use our shell, they need to look for
tutorials on the POSIX shell. Align descriptions through out the tree
with this reality, consistent with it's manual and common parlance.

We made a similar change to the doc tree in b4d6eb01540fe.

MFC after: 3 days

[carlavilla]

As you said, we made this change in docs too

[ziaee]

Thanks @carlavilla !

[imp]

So I approve of this belatedly.

4BSD had csh in it, and v7 had a new Bourne shell (as opposed to the Johnson shell from v6 and a few other weird shells knocking around bell labs that are lost to the mists of time and/or the TUHS mailing list, or the Korn Shell, which was another POSIX-ish shell). Bourne shell was a synonym for POSIX shell for a number of years, and we used it to differentiate between csh and sh. They are still interchangeable, but I've noticed a push to use POSIX compatible shell scripts to differentiate them from shell scripts that use Bash features from those that don't. korn shell or ksh is distinctive enough that people talk about it using the specific name still, which while ambiguous with the band name, is sufficiently specific for the context given this picture https://www.reddit.com/r/linuxmasterrace/comments/j2w8rl/a_korn_shell_ksh_manual_being_signed_by_members/