syscall: add tty support to StartProcess

These changes add a Ctty int field to the Unix syscall.ProcAttr which,
if set >= 0 in conjuction with Setsid=true, will be used by
forkAndExecInChild as the file descriptor for the new child's
controlling terminal.

Necessary changes have been made to mkerrors.sh to generate defs for
TIOC*, though changes to its output files are not included here.
The changes made should support Linux, FreeBSD and Darwin, at least.

R=iant, bradfitz, r, rsc, borman
CC=golang-dev
https://golang.org/cl/4532075

src/pkg/syscall/mkerrors.sh

@@ -62,6 +62,7 @@ includes_Darwin='
 #include <netinet/in.h>
 #include <netinet/ip.h>
 #include <netinet/ip_mroute.h>
+#include <termios.h>
 '
 
 includes_FreeBSD='
@@ -71,6 +72,7 @@ includes_FreeBSD='
 #include <sys/sockio.h>
 #include <sys/sysctl.h>
 #include <sys/wait.h>
+#include <sys/ioctl.h>
 #include <net/bpf.h>
 #include <net/if.h>
 #include <net/if_types.h>
@@ -145,6 +147,7 @@ done
 		$2 !~ "NLA_TYPE_MASK" &&
 		$2 ~ /^(NETLINK|NLM|NLMSG|NLA|IFA|RTM|RTN|RTPROT|RTA|RTAX|RTNH|ARPHRD|ETH_P)_/ ||
 		$2 ~ /^SIOC/ ||
+		$2 ~ /^TIOC/ ||
 		$2 ~ /^(IFF|IFT|NET_RT|RTM|RTF|RTV|RTA|RTAX)_/ ||
 		$2 ~ /^BIOC/ ||
 		$2 !~ /^(BPF_TIMEVAL)$/ &&