Log only one message for dangerous log settings.

We log only one message, containing a complete list of what's
wrong.  We log the complete list whenever any of the possible things
that could have gotten wrong gets worse.

Fix for #9870. Bugfix on 10480dff01bece13fab, which we merged in
0.2.5.1-alpha.
Merge remote-tracking branch 'public/bug9870'

Conflicts:
	src/or/config.c