Refactor UseEntryNodes so the original configured value is preserved

Parse the value to UseEntryNodes_option, then set UseEntryNodes before
validating options.

This way, Authorities, Tor2web, and Single Onion Services don't write
spurious "UseEntryNodes 0" lines to their configs. Document the fact that
these tor configurations ignore UseEntryNodes in the manual page.

Also reorder options validation so we modify UseEntryNodes first, then
check its value against EntryNodes.

And silence a warning about disabled UseEntryNodes for hidden services
when we're actually in non-anonymous single onion service mode.