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/compat.h

@@ -96,4 +96,14 @@ bool static inline IsSelectableSocket(const SOCKET& s) {
 #endif
 }
 
+// MSG_NOSIGNAL is not available on some platforms, if it doesn't exist define it as 0
+#if !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(MSG_DONTWAIT)
+#define MSG_DONTWAIT 0
+#endif
+
 #endif // BITCOIN_COMPAT_H

src/net.cpp

@@ -11,6 +11,7 @@
 
 #include <banman.h>
 #include <clientversion.h>
+#include <compat.h>
 #include <consensus/consensus.h>
 #include <crypto/sha256.h>
 #include <net_permissions.h>
@@ -72,16 +73,6 @@ static constexpr std::chrono::seconds MAX_UPLOAD_TIMEFRAME{60 * 60 * 24};
 // We add a random period time (0 to 1 seconds) to feeler connections to prevent synchronization.
 #define FEELER_SLEEP_WINDOW 1
 
-// MSG_NOSIGNAL is not available on some platforms, if it doesn't exist define it as 0
-#if !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(MSG_DONTWAIT)
-#define MSG_DONTWAIT 0
-#endif
-
 /** Used to pass flags to the Bind() function */
 enum BindFlags {
     BF_NONE         = 0,

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);