Only build net/hook_cloexec.go on GNU/Linux and FreeBSD, because those
    are the only systems with accept4.
    
    Add syscall/libcall_bsd.go to define sendfile for *BSD and Solaris.
    
    Revert tcpsockopt_solaris.go back to the earlier version, so that it
    works on Solaris 10.
    
    Always pass the address of a Pid_t value to TIOCGPGRP and TIOCSPGRP.
    
    Include <unistd.h> in runtime/go-varargs.c.
    
    Reviewed-on: https://go-review.googlesource.com/16719

From-SVN: r229880