This patch adds pipe(). The pipes are built using the same FIFO implementation,
"af_local_buffer", as used by the existing unix-domain socketpair
implementation - while the socket-pair used two of these buffers, a pipe
uses one.

This implementation deviates from traditional POSIX pipe behavior in two
ways that we should fix in followup-patches:

1. SIGPIPE is not supported: A write to a pipe whose read end is closed
   will always return EPIPE, and not generate a SIGPIPE signal.
   Programs that rely on SIGPIPE will break, but SIGPIPE is completely out
   of fashion, and normally ignored.

2. Unix-style "atomic writes" are not obeyed. A write(), even if smaller
   than PIPE_BUF (=4096 on Linux, whose ABI we're emulating), may partially
   succeed if the pipe's buffer is nearly full. Only a write() of a single
   byte is guaranteed to be atomic.

   We hope that Java doesn't rely on multi-byte write() atomicity
   (single-byte writes are enough for waking poll, for example), and users
   of Java's "Pipe" class definitely can't (as Java is not Posix-only),
   so we hope this will not cause problems. Fixing this issue (which is easy)
   is left as a TODO in the code.

Additionally, this patch marks with a FIXME (but doesn't fix) a serious
bug in the code's iovec handling, so writev() and readv() are expected
not to work in this version of pipe() - and also on the existing socketpair.