doco socket

[Kevin Ryde]

        * posix.texi (Network Sockets and Communication): In socket, use
        @defvar for protocol variables, cross reference for getprotobyname,
        note it's usually connect and accept that establishes communication.

 - Scheme Procedure: socket family style proto
 - C Function: scm_socket (family, style, proto)
     Return a new socket port of the type specified by FAMILY, STYLE
     and PROTO.  All three parameters are integers.  The possible
     values for FAMILY are as follows, where supported by the system

      - Variable: PF_UNIX
      - Variable: PF_INET
      - Variable: PF_INET6

     The possible values for STYLE are as follows, again where
     supported by the system,

      - Variable: SOCK_STREAM
      - Variable: SOCK_DGRAM
      - Variable: SOCK_RAW

     PROTO can be obtained from a protocol name using `getprotobyname'
     (*note Network Databases::).  A value of zero specifies the
     default protocol, which is usually right.

     A socket cannot by used for communication until it has been
     connected somewhere, usually with either `connect' or `accept'
     below.