net: dedup MSG_NOSIGNAL and MSG_DONTWAIT definitions

Deduplicate `MSG_NOSIGNAL` and `MSG_DONTWAIT` definitions from `net.cpp`
and `netbase.cpp` to `compat.h` where they can also be reused by other
code.

src/netbase.cpp

@@ -5,6 +5,7 @@
 
 #include <netbase.h>
 
+#include <compat.h>
 #include <sync.h>
 #include <tinyformat.h>
 #include <util/sock.h>
@@ -30,10 +31,6 @@
 #include <poll.h>
 #endif
 
-#if !defined(MSG_NOSIGNAL)
-#define MSG_NOSIGNAL 0
-#endif
-
 // Settings
 static Mutex g_proxyinfo_mutex;
 static proxyType proxyInfo[NET_MAX] GUARDED_BY(g_proxyinfo_mutex);