handle excludenodes for dir fetch/post

handle excludenodes for dir fetch/post

If we're picking a random directory node, never pick an excluded one.
But if we've chosen a specific one (or all), allow it unless strictnodes
is set (in which case warn so the user knows it's their fault).

When warning that we won't connect to a strictly excluded node,
log what it was we were trying to do at that node.

When ExcludeNodes is set but StrictNodes is not set, we only use
non-excluded nodes if we can, but fall back to using excluded nodes
if none of those nodes is usable.