MSG_PROXY for GNU/Hurd (previously we believed that

MSG_PROXY for GNU/Hurd (previously we believed that
all GNU libc platforms have MSG_PROXY.  Untrue).
In fact this ended up as a major MSG_* and SCM_*
update.  The MSG_XXX known to be enums in some
versions of the glibc are now probed for and respective
HAS_MSG_XXX are defined.  While I was at it I noticed
SCM_RIGHTS being similarly an enum.  This reminded me of
an ancient discussion in perl5-porters:
http://www.xray.mpe.mpg.de/mailing-lists/perl5-porters/9612/msg01017.html
The BSD socket interface has a nifty feature for passing
file descriptors and credentials--via sockets.  It may be
too late to add this functionality to the CORE but
at least Configure now probes for the functions,
structs, and includes, defining the appropriate
HAS_YYY and I_ZZZ, and the Socket extension exports
the constants, in case somebody wants to write an
extension for this interface.

p4raw-id: //depot/cfgperl@2285