In ovsdb-server, the OVSDB log code is used to open the databases specified
on the command line before ovsdb-server daemonizes itself.  Afterward, it
is occasionally necessary for ovsdb-server to reference those files by name
again.  When that happens, if daemonization changed the current directory
to the root, any relative names are no longer valid and those references
will fail.  Until now, this was handled at a higher level in ovsdb-server,
but in the future it will be convenient to handle it in the log code
itself.  This commit prepares for that by making the log code take the
absolute name of log files itself.

Signed-off-by: Ben Pfaff <blp@ovn.org>