salt.utils: add is_linux() and is_windows()
salt.utils: add is_linux() and is_windows()

These were mainly added after I saw the 2/3 different ways various
places in the salt code check to see if a platform is indeed windows
or not and I wanted to consolidate them all including the grains before
there necessarily is __grains__['os'] to rely on.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt