Convert closesocket 'compat wrapper' to function in netbase

Simpler alternative to #4348.

The current setup with closesocket() is strange. It poses
as a compatibility wrapper but adds functionality.

Rename it and make it a documented utility function in netbase.

Code movement only, zero effect on the functionality.

src/compat.h

@@ -59,19 +59,4 @@ typedef u_int SOCKET;
 #define SOCKET_ERROR        -1
 #endif
 
-inline int myclosesocket(SOCKET& hSocket)
-{
-    if (hSocket == INVALID_SOCKET)
-        return WSAENOTSOCK;
-#ifdef WIN32
-    int ret = closesocket(hSocket);
-#else
-    int ret = close(hSocket);
-#endif
-    hSocket = INVALID_SOCKET;
-    return ret;
-}
-#define closesocket(s)      myclosesocket(s)
-
-
 #endif