vlog: Make vlog initialize itself when necessary.

It's more convenient if clients don't have to initialize modules
explicitly.

The most important part of this change is to initialize the default
log levels statically.  Previously, by initializing log levels only
from vlog_init(), all the log levels appeared to be VLL_EMER (0) if
vlog_init() was accidentally not called at all.  This was not intended
behavior, so this commit fixes it.

This commit also fixes up a few test programs whose tests accidentally
depended on this behavior, by making them explicitly turn off log
messages that were implicitly turned off before.