1. Three bugs reported by Mike Schiraldi:
    * If -fautolink is turned on, a naked @ becomes a mailto:
       link.  Fixed by only triggering `maybe_tag_or_link()` on
       alphabetic characters.
    * If -fnohtml is set, forced linebreaks become <br/>.
       Fixed by filtering out nonprinting nonwhitespace characters
       on the input and using ^C as the <br/> generator character
       instead of converting the forced linebreak directly to <br/>.
    * inline links (via []()) don't allow spaces or escaped ) characters.
       Fixed by rewriting `linkyurl()` so it builds a new `Cstring` for
       link.link (which needs to be deleted after use, so I added a new
       `deallocate` field to the bookmark record for `linkylinky` to use,)
       changing the link gobbler to absorb input until a terminator instead
       of until whitespace or a terminator) and allowing \ to escape ), =,
       or ".
2. Change the `Qchar()` function so that it take an integer argument instead
   of a char.  (reported by Josh Wood from the plan9 port.)
3. Some bugs I discovered
    *  Flag the raw: pseudo-protocol so that it will be disabled when
       DENY_HTML is set.
    *  Rework `maybe_tag_or_link()` so it better rejects random stuff
       that begins with a `<`.
4.  Add the new `abbr:` pseudo-protocol (the html `<abbr>` tag)