syscall: add TCIOFLUSH family of constants

Fixes #6355.

zerrors_linux_{386,amd64,arm}.go were regenerated using mkerrors.sh but I opted to add the three TC.*FLUSH lines by hand to keep the diff smaller and avoid problems with the API checker.

I'll check freebsd and darwin, could I ask for help with net/open bsd.

R=mikioh.mikioh, jsing, minux.ma, bradfitz
CC=golang-dev
https://golang.org/cl/13660043

src/pkg/syscall/consistency_unix_test.go

@@ -23,3 +23,12 @@ func _() {
 		_ int = syscall.PRIO_PGRP
 	)
 }
+
+// termios functions and constants
+func _() {
+	const (
+		_ int = syscall.TCIFLUSH
+		_ int = syscall.TCIOFLUSH
+		_ int = syscall.TCOFLUSH
+	)
+}

src/pkg/syscall/mkerrors.sh

@@ -107,6 +107,7 @@ includes_Linux='
 #include <net/if_arp.h>
 #include <net/route.h>
 #include <netpacket/packet.h>
+#include <termios.h>
 
 #ifndef MSG_FASTOPEN
 #define MSG_FASTOPEN    0x20000000

src/pkg/syscall/zerrors_linux_386.go

@@ -1084,6 +1084,9 @@ const (
 	S_IXGRP                          = 0x8
 	S_IXOTH                          = 0x1
 	S_IXUSR                          = 0x40
+	TCIFLUSH                         = 0x0
+	TCIOFLUSH                        = 0x2
+	TCOFLUSH                         = 0x1
 	TCP_CONGESTION                   = 0xd
 	TCP_CORK                         = 0x3
 	TCP_DEFER_ACCEPT                 = 0x9

src/pkg/syscall/zerrors_linux_amd64.go

@@ -1085,6 +1085,9 @@ const (
 	S_IXGRP                          = 0x8
 	S_IXOTH                          = 0x1
 	S_IXUSR                          = 0x40
+	TCIFLUSH                         = 0x0
+	TCIOFLUSH                        = 0x2
+	TCOFLUSH                         = 0x1
 	TCP_CONGESTION                   = 0xd
 	TCP_CORK                         = 0x3
 	TCP_DEFER_ACCEPT                 = 0x9

src/pkg/syscall/zerrors_linux_arm.go

@@ -1095,6 +1095,9 @@ const (
 	S_IXGRP                          = 0x8
 	S_IXOTH                          = 0x1
 	S_IXUSR                          = 0x40
+	TCIFLUSH                         = 0x0
+	TCIOFLUSH                        = 0x2
+	TCOFLUSH                         = 0x1
 	TCP_CONGESTION                   = 0xd
 	TCP_CORK                         = 0x3
 	TCP_DEFER_ACCEPT                 = 0x9