wip/fsync branch for review

[Colin Walters <walters verbum org>]

Hi, so I was hitting an issue with the Fedora syslinux where it was 
unable to find the bootloader config.

Now at present, we have a global sync() call, but I think that may not 
be enough; the man page does say it won't flush the journal.  Anyways, 
this caused me to decide that I need to carefully fsync()/fdatasync() 
*everything* to disk.

Theoretically for some cases journaling should work, but in other cases 
ostree was straight up missing calls to e.g. fdatasync() on the .conf 
files.

Another thing we need to do is call fsync() on the *directory* fd when 
we're done writing entries. 

Since ostree is in the position where it is *the only* userspace 
process writing to disk, there's really no excuse for not Doing It 
Right.

Hence:
https://git.gnome.org/browse/ostree/log/?h=wip/fsync

Review appreciated!