Replace the hacky 'OSSocket' type with a closure.

The mechanism for constructing a new connection-type Socket when a
listening one receives an incoming connection previously worked by
passing a platform-specific 'OSSocket' type to the plug_accepting
function, which would then call sk_register to wrap it with a proper
Socket instance. This is less flexible than ideal, because it presumes
that only one kind of OS object might ever need to be turned into a
Socket. So I've replaced OSSocket throughout the code base with a pair
of parameters consisting of a function pointer and a context such that
passing the latter to the former returns the appropriate Socket; this
will permit different classes of listening Socket to pass different
function pointers.

In deference to the reality that OSSockets tend to be small integers
or pointer-sized OS handles, I've made the context parameter an
int/pointer union that can hold either of those directly, rather than
the usual approach of making it a plain 'void *' and requiring a
context structure to be dynamically allocated every time.

[originally from svn r10068]

unix/unix.h

@@ -24,9 +24,6 @@ struct FontSpec *fontspec_new(const char *name);
 
 typedef void *Context;                 /* FIXME: probably needs changing */
 
-typedef int OSSocket;
-#define OSSOCKET_DEFINED	       /* stop network.h using its default */
-
 extern Backend pty_backend;
 
 typedef uint32_t uint32; /* C99: uint32_t defined in stdint.h */