If we pass XATTR_REPLACE then the attribute must already exist, which
is not our intent. Passing zero creates the attribute if necessary,
or replaces it when it already exists.
https://bugzilla.gnome.org/show_bug.cgi?id=705893
* Specifying global options after the command for a more natural:
# ostree commit --repo=/path/to/repo ...
* Support asking for --help without --repo
# ostree commit --help
* Support short form of -h
* Support specifying --repo without equals sign
# ostree --repo /path/to/repo commit ...
* Support global --help and -h
# ostree --help
* Ditto for ostree admin sub commands
* Removed some leaky code
https://bugzilla.gnome.org/show_bug.cgi?id=705903