This was just an alias for "unix" that I added early on back when there
was some belief that we might be compatible with OpenBSD. We're clearly
never going to be compatible with their pledges so just drop the alias.
Now that WindowServer broadcasts the system theme using an anonymous
file, we need clients to pledge "recvfd" so they can receive it.
Some programs keep the "shared_buffer" pledge since it's still used for
a handful of things.
