Replace DirPort_set with dir_server_mode

legacy/trac#12538 (moved) replaces most uses of DirPort_set with dir_server_mode, legacy/trac#4483 (moved) adds another use of DirPort_set.

I don't know what order they'll merge to master, if legacy/trac#12538 (moved) merges first, we'll need to replace the DirPort_set in legacy/trac#4483 (moved) with dir_server_mode.

(There might be commit conflicts due to these changes.)