vlog: Stop using explicit references to external log modules.

vlog: Stop using explicit references to external log modules.

It's always risky to write "extern" declarations outside a header file,
since there's no way to ensure the type of what's being referenced is
correct.  In these cases, we can easily avoid the extern reference, so do
so.

There is a little tradeoff here, in that referring to the log modules
through strings means that we catch an incorrect module name at runtime
instead of at link time, but I think that the risk here is minimal because
the mistake will be found by every test in "make check" that runs any of
the utilities, since they make these calls as one of their first tasks
during initialization.

Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Russell Bryant <russell@ovn.org>