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

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.

[tedbo, 2010-03-18 23:45:45]

LGTM

I don't want to hold up the gpt work, so this looks ok. I do think that
exclusive mode should not be the default and instead there should require a flag
to open in shared mode.

http://codereview.chromium.org/1133004/diff/1/3
File gpt.8 (right):

http://codereview.chromium.org/1133004/diff/1/3#newcode90
gpt.8:90: default is to open in shared mode, which can be dangerous. Of course,
Maybe the default should be to open in exclusive mode and the flag should
override that instead?