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

core/sys/posix/netinet_tcp.odin

@@ -1,3 +1,4 @@
+#+build linux, darwin, netbsd, openbsd, freebsd
 package posix
 
 // netinet/tcp.h - definitions for the Internet Transmission Control Protocol (TCP)
@@ -6,6 +7,4 @@ when ODIN_OS == .Darwin || ODIN_OS == .FreeBSD || ODIN_OS == .NetBSD || ODIN_OS
 
 	TCP_NODELAY :: 0x01
 
-} else {
-	#panic("posix is unimplemented for the current target")
 }