compat: use `unsigned int` instead of `u_int`

`u_int` is not available on some platforms (not sure what standard it's
supposed to be part of), we don't use it anywhere else, and it doesn't
hurt to simply write `unsigned int` out here.

src/compat.h

@@ -48,7 +48,7 @@
 #endif
 
 #ifndef WIN32
-typedef u_int SOCKET;
+typedef unsigned int SOCKET;
 #include "errno.h"
 #define WSAGetLastError()   errno
 #define WSAEINVAL           EINVAL