Use syscall.TIOCGPTN and syscall.TIOCSPTLCK for ioctl

This PR changes the ioctl related code to use constants defined in the syscall package instead of calculating them.

On Linux ppc64 and ppc64le, the constants are different from the calculated value. The constants in the syscall package are valid even on  ppc64 and ppc64le. I confirmed that these syscall constants are defined in go 1.2.1.

I am sorry, but I forgot to include this change in my previous PR #25.

Signed-off-by: Yohei Ueda <yohei@jp.ibm.com>

Might as well use syscall.TIOCSPTLCK directly in the rest of the file and get rid of ioctl_TIOCSPTLCK. Same goes for ioctl_TIOCGPTN.