build: cleanup: define MSG_DONTWAIT/MSG_NO_SIGNAL locally

Define MSG_DONTWAIT and MSG_NO_SIGNAL in the implementation files that
use them (`net.cpp` and `netbase.cpp`), instead of compat.h which is
included all over the place.

This avoids putting them in the global namespace, as defining them as 0
is a hack that works for our specific usage, but it is not a general
solution.

Also makes sure they are defined only once so the `!defined(MSG_x)` guard can go.

src/compat.h

@@ -72,16 +72,6 @@ typedef u_int SOCKET;
 #define MAX_PATH            1024
 #endif
 
-// As Solaris does not have the MSG_NOSIGNAL flag for send(2) syscall, it is defined as 0
-#if !defined(HAVE_MSG_NOSIGNAL) && !defined(MSG_NOSIGNAL)
-#define MSG_NOSIGNAL 0
-#endif
-
-// MSG_DONTWAIT is not available on some platforms, if it doesn't exist define it as 0
-#if !defined(HAVE_MSG_DONTWAIT) && !defined(MSG_DONTWAIT)
-#define MSG_DONTWAIT 0
-#endif
-
 #if HAVE_DECL_STRNLEN == 0
 size_t strnlen( const char *start, size_t max_len);
 #endif // HAVE_DECL_STRNLEN