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 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-server: add new-and-improved APIs

Add new APIs to SoupServer, allowing multiple listening sockets on a
server, and listening on an existing fd or GSocket, and with better
error handling.

https://bugzilla.gnome.org/show_bug.cgi?id=522519
https://bugzilla.gnome.org/show_bug.cgi?id=561547
https://bugzilla.gnome.org/show_bug.cgi?id=621138
tests: port old tests to new SoupServer APIs, add some new tests
SoupServer: add deprecation tags to deprecated API