Stop using O_EXCL by default when opening the device for writing.

Using O_EXCL prevents the gpt tool from being able to modify any part of the
GPT while there is a mounted filesystem on that drive. We need to be able to
modify it so we can mark a newly upgraded image as bootable.

Also updated the man page a bit, although it's still way out of sync.

Review URL: http://codereview.chromium.org/1133004
3 files changed