socketmodule.c.  No code outside of the .c file references it, so it
doesn't belong the .h file (at least not yet ...), and declaring it
an imported symbol in the .h file can't be made to work on Windows (it's
a cross-DLL symbol then) without substantial code rewriting.  Also
repaired the comment that goes along with the decl, to stop referring
to names and functions that haven't existed for 7 years <wink>.

socketmodule.c compiles cleanly on Windows again.  The test_socket dies
at once, though (later).