Proposal to obsolete SYS_pipe

[Kamil Rytarowski <krytarowski%gmail.com@localhost>]

I propose to deprecate SYS_pipe.

It is a special syscall that returns two integers from one function
call. Fanciness is not compatible with regular C syntax and it demands
per-cpu assembly wrappers and rump-kernel workarounds. It's not easily
usable with syscall(2).

OpenBSD and FreeBSD already deprecated this traditional pipe(2) syscall
replacing it with a more portable equivalent in C.

Changes:
 - I've marked pipe(2) as compat_80.
 - I've removed generation of garbage in retval[2] from pipe2(2).
 - I've reimplemented in C the pipe(2) syscall with pipe2(2).
 - I've adjusted the surrounding code for the changes.

http://netbsd.org/~kamil/patch-00039-obsolete-SYS_pipe.txt