This is to make it easier to support Solaris syslog.  On
Solaris syslog messages are sent via STREAMS using putmsg to
/dev/conslog.  The putmsg call uses a a control buffer of type
log_cdtl and a data buffer which is the message, and it is in
general a big mess.  This CL just splits out the Unix domain
support so that Solaris can use a different mechanism.  I do
not propose to implement the Solaris support today.  This
split will make it possible for gccgo to just call the libc
function for now.

R=r, rsc
CC=golang-dev
https://golang.org/cl/4261061