disable-shell.inc: add oksh

[kmk3]

"Portable OpenBSD ksh, based on the Public Domain Korn Shell (pdksh)."

Project page: https://github.com/ibara/oksh

$ pacman -Q oksh
oksh 6.8.1-1
$ pacman -Qlq oksh | grep bin/
/usr/bin/
/usr/bin/oksh

[rusty-snake]

Thanks for this contribution!

[kmk3]

Hmm I noticed that GitHub butchered the author name and author date of the
original commit.

@rusty-snake Did you use rebase-merge or something like that? I think I'll
test later what option causes this.

P.S.: It also set the committer name to simply "GitHub". IMO ideally it would
be set to whoever merged it, like in a normal non-fast-forward merge, though
that's secondary.

[rusty-snake]

Did you use rebase-merge or something like that?

"squash and merge"