Merge #4612

4612: Handle SIGINT and SIGTERM (fixes #4610) r=heinrich5991 a=def-

Code by Robyt3 taken from Teeworlds, added SIGTERM

## Checklist

- [x] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test if it works standalone, system.c especially
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: Robert Müller <robert.mueller@uni-siegen.de>
Co-authored-by: Dennis Felsing <dennis@felsin9.de>

src/engine/server/server.cpp

@@ -48,6 +48,10 @@
 #include <windows.h>
 #endif
 
+#include <signal.h>
+
+volatile sig_atomic_t InterruptSignaled = 0;
+
 CSnapIDPool::CSnapIDPool()
 {
 	Reset();
@@ -2714,6 +2718,11 @@ int CServer::Run()
 
 				PacketWaiting = x > 0 ? net_socket_read_wait(m_NetServer.Socket(), x) : true;
 			}
+			if(InterruptSignaled)
+			{
+				Console()->Print(IConsole::OUTPUT_LEVEL_STANDARD, "server", "interrupted");
+				break;
+			}
 		}
 	}
 	const char *pDisconnectReason = "Server shutdown";
@@ -3596,6 +3605,15 @@ void CServer::SnapSetStaticsize(int ItemType, int Size)
 
 static CServer *CreateServer() { return new CServer(); }
 
+void HandleSigIntTerm(int Param)
+{
+	InterruptSignaled = 1;
+
+	// Exit the next time a signal is received
+	signal(SIGINT, SIG_DFL);
+	signal(SIGTERM, SIG_DFL);
+}
+
 int main(int argc, const char **argv) // ignore_convention
 {
 	cmdline_fix(&argc, &argv);
@@ -3624,6 +3642,9 @@ int main(int argc, const char **argv) // ignore_convention
 		return -1;
 	}
 
+	signal(SIGINT, HandleSigIntTerm);
+	signal(SIGTERM, HandleSigIntTerm);
+
 	CServer *pServer = CreateServer();
 	IKernel *pKernel = IKernel::Create();