os: add ModeType constant to mask file type bits
os: add ModeType constant to mask file type bits

This covers the lack of IsRegular comfortably:

    if stat.Mode()&os.ModeType == 0 { ... }

R=golang-dev, r, rsc, r, gustavo
CC=golang-dev
http://codereview.appspot.com/5440075
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go