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