libgo: Solaris portability fixes.

libgo: Solaris portability fixes.

    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