network: icmp emulation (#694)

## Link to GitHub Issue or related Pull Request, if one exists
#0

## Description of change
Add `-icmphook` option under spicecfg Development/Network for emulating
keepalive ping replies in user mode, so games do not need to open
privileged raw ICMP sockets. This makes connecting to networks possible
on macOS under Whisky, and on Windows with ICMP firewalled or spice
running without Administrator privileges.

## Testing
Tested DDR and SDVX on Windows 10 and macOS Tahoe. Network check status
in test menu shows `CONNECTED` when -icmphook is enabled, and `NOT
CONNECTED` when it is disabled. Actual network functionality works as
expected (under previously impossible conditions).

src/spice2x/hooks/icmphook_net.h

@@ -0,0 +1,13 @@
+#pragma once
+
+#include <winsock2.h>
+
+bool icmphook_is_emulated_socket(SOCKET s);
+
+/*!
+ * Handle bind() for emulated ICMP sockets: records interface address and succeeds without kernel bind.
+ * Returns true if this socket was handled (caller should return 0).
+ */
+bool icmphook_try_bind(SOCKET s, const struct sockaddr *name, int namelen, int *out_result);
+
+void icmphook_net_init();