doc ipv4 address conversions

[Kevin Ryde]

I think these could be described under the ipv4 address section,
rather than just under "bind".

        * posix.texi (Network Address Conversion): Describe numeric
        representation in Guile.  Add INADDR_LOOPBACK and INADDR_BROADCAST,
        add commented-out INADDR_NONE.

Describing the number format doesn't give anything new away, since the
examples have implicitly shown it.


IPv4 Address Conversion
.......................

An IPv4 Internet address is a 4-byte value, represented in Guile as an
integer in network byte order (meaning the first byte is the most
significant in the number).

 - Variable: INADDR_LOOPBACK
     The address of the local host using the loopback device, ie.
     `127.0.0.1'.

 - Variable: INADDR_BROADCAST
     The broadcast address on the local network.

@c  INADDR_NONE is defined in the code, but serves no purpose.
@c  inet_addr() returns it as an error indication, but that function
@c  isn't provided, for the good reason that inet_aton() does the same
@c  job and gives an unambiguous error indication.  (INADDR_NONE is a
@c  valid 4-byte value, in glibc it's the same as INADDR_BROADCAST.)
@c
@c  @defvar INADDR_NONE
@c  No address.
@c  @end defvar