Revise proposal 185 to make DirCache flag unconditional

Based on review by Karsten, who says:

"""I'm not sure why a) relays with a working DirPort shouldn't
   include "dir-cache 1" in their router descriptor and b)
   authorities shouldn't assign the "DirCache" flag to relays with a
   working DirPort that don't have a "dir-cache 1" line in their
   router descriptor.  I understand that neither of the two actions
   are necessary to make the proposal work.  But this could be a
   chance to get rid of the DirPort concept entirely and only rely
   on "dir-cache 1" and "DirCache" flags in the future."""