1. "autolinks" -- expand unbracketed urls into links
      (flag MKD_AUTOLINK, -fautolink in the markdown program)
   2. "safe links" -- don't linkify footnotes where the link protocol
      isn't a well-known protocol (currently http: https:,ftp:,news:)
      (flag MKD_SAFELINK, -fsafelink in the markdown program)

Also do a couple of performance and sanity tweaks:
   1. Reorder the logic of maybe_tag_or_link() to process mailto: before
      other link types, and explicitly if() ... else the control flow
   2. make the protocols[] array into an name/size structure, so I don't
      have to spend time doing strlen() when running isautoprotocol().