Add soup-socket-private.h

SoupSocket-related stuff made up more than half of
soup-misc-private.h. Split it out into its own header.
soup-socket: Add SoupSocketProperties to transfer props from session to socket

There are a large number of SoupSession properties that need to be
propagated through the SoupConnection to the SoupSocket. Wrap them all
up into a single struct for ease of carrying.
soup-socket: add flag close_on_dispose to SoupSocket

https://bugzilla.gnome.org/show_bug.cgi?id=721343
soup-socket: add properties to create a SoupSocket from an fd or GSocket
soup-socket: misc improvements