Rewrite __fish_print_help to produce better results

As a result of this rewrite, the output now:

* Expands to fit the terminal width, like `man` does
* Preprocesses the manpage with `tbl` just in case, since `man` does
  this, even though I doubt any fish manpages use `tbl` formatting.
* Handle bold/underline with the `ul` command as it was designed for
  instead of trying to fake it with `sed`.
* Compresses blank lines as `man` does with the default `less -is`
  pager.
A little extra tweaking so builtin_print_help looks better

The terminal width magic that __fish_print_help learned doesn't help
when builtin_print_help runs it in a subshell. Instead, add an
undocumented --tty-width flag to __fish_print_help that's used to pass
the terminal width.