doco socketpair

[Kevin Ryde]

        * posix.texi (Network Sockets and Communication): In socketpair,
        clarify the return is a pair with ports in car and cdr, note
        connection is full duplex, refer to `socket' for parameters, refer
        to PF_UNIX rather than AF_UNIX.

The glibc manual has AF_LOCAL (the same as AF_UNIX) in its description
of socketpair, but if I understand this stuff it's PF_UNIX (or
PF_LOCAL) which is correct for a protocol namespace (as opposed to an
address format).  I guess the two are the same value anyway, so it
doesn't matter.


 - Scheme Procedure: socketpair family style proto
 - C Function: scm_socketpair (family, style, proto)
     Return a pair, the `car' and `cdr' of which are two unnamed socket
     ports connected to each other.  The connection is full-duplex, so
     data can be transferred in either direction between the two.

     FAMILY, STYLE and PROTO are as per `socket' above.  But many
     systems only support socket pairs in the `PF_UNIX' family.  Zero
     is likely to be the only meaningful value for PROTO.