Fix O_SYNC paramter definitions on Linux x86.

The parameter values changed with kernel 2.6.33. Quoting the header
file, "only O_DSYNC semantics were implemented [before], but using
the O_SYNC flag. We continue to use the existing numerical value
for O_DSYNC semantics now, [...]".
Merge pull request #630 from klickverbot/fcntl-x86-linux

Fix O_SYNC paramter definitions on Linux x86.