unix: don't redefine constants already defined in glibc headers

These are all present in glibc 2.37 which is currently used to generate
these constants.

Change-Id: I3ffbd8f8581e85e8854222ad3f2c7f44aac106c7
Reviewed-on: https://go-review.googlesource.com/c/sys/+/552417
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>

Author: tklauser

unix/mkerrors.sh

@@ -284,10 +284,6 @@ struct ltchars {
 #include <asm/termbits.h>
 #endif
 
-#ifndef MSG_FASTOPEN
-#define MSG_FASTOPEN    0x20000000
-#endif
-
 #ifndef PTRACE_GETREGS
 #define PTRACE_GETREGS	0xc
 #endif
@@ -296,14 +292,6 @@ struct ltchars {
 #define PTRACE_SETREGS	0xd
 #endif
 
-#ifndef SOL_NETLINK
-#define SOL_NETLINK	270
-#endif
-
-#ifndef SOL_SMC
-#define SOL_SMC 286
-#endif
-
 #ifdef SOL_BLUETOOTH
 // SPARC includes this in /usr/include/sparc64-linux-gnu/bits/socket.h
 // but it is already in bluetooth_linux.go
@@ -320,11 +308,6 @@ struct ltchars {
 #undef TIPC_WAIT_FOREVER
 #define TIPC_WAIT_FOREVER 0xffffffff
 
-// Copied from linux/l2tp.h
-// Including linux/l2tp.h here causes conflicts between linux/in.h
-// and netinet/in.h included via net/route.h above.
-#define IPPROTO_L2TP		115
-
 // Copied from linux/netfilter/nf_nat.h
 // Including linux/netfilter/nf_nat.h here causes conflicts between linux/in.h
 // and netinet/in.h.