[NaCk SDK] nacl_io: Don't assume ~S_IFMT are the mode bits.

[NaCk SDK] nacl_io: Don't assume ~S_IFMT are the mode bits.

Define a new macro S_MODEBITS to represent the
mode bits settable by chmod/fchmod.

Add test to verify that file type cannot be changed
with chmod.

Committed: https://crrev.com/b979b669be38c55ba28a3562a310edf3b9403511
Cr-Commit-Position: refs/heads/master@{#300341}

[Sam Clegg, 2014-10-20 18:14:55]

sbc@chromium.org changed reviewers:
+ binji@chromium.org

[Sam Clegg, 2014-10-20 18:14:55]

sbc@chromium.org changed reviewers:
+ binji@chromium.org

[binji, 2014-10-20 18:18:48]

lgtm

[Sam Clegg, 2014-10-20 20:29:09]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2014-10-20 20:30:54]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/671513002/1

[commit-bot: I haz the power, 2014-10-20 21:49:41]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-10-20 21:50:17]

Patchset 1 (id:??) landed as
https://crrev.com/b979b669be38c55ba28a3562a310edf3b9403511
Cr-Commit-Position: refs/heads/master@{#300341}