sys/posix: impl rest of linux, impl some of Windows

core/sys/posix/netinet_in.odin

@@ -1,3 +1,4 @@
+#+build linux, darwin, netbsd, openbsd, freebsd
 package posix
 
 import "core:c"
@@ -61,6 +62,11 @@ when ODIN_OS == .Darwin || ODIN_OS == .FreeBSD || ODIN_OS == .NetBSD || ODIN_OS
 			sin6_scope_id: c.uint32_t,  /* [PSX] set of interfaces for a scope */
 		}
 
+		ipv6_mreq :: struct {
+			ipv6mr_multiaddr: in6_addr, /* [PSX] IPv6 multicast address */
+			ipv6mr_interface: c.uint,   /* [PSX] interface index */
+		}
+
 		IPV6_MULTICAST_IF   :: 17
 		IPV6_UNICAST_HOPS   :: 16
 		IPV6_MULTICAST_HOPS :: 18
@@ -223,6 +229,4 @@ when ODIN_OS == .Darwin || ODIN_OS == .FreeBSD || ODIN_OS == .NetBSD || ODIN_OS
 		)
 	}
 
-} else {
-	#panic("posix is unimplemented for the current target")
 }