__fish_print_help: use man as-is

Since 0fea1dae8c (__fish_print_help: Make formatting more man-like,
2024-10-03), there is barely any difference left between "man abbr"
and "abbr -h".

The main difference is that it looks almost like man but is actually
nroff/mandoc and less.  This means it doesn't support environment
variables like MANWIDTH and possibly others.

Let's use full "man" for now.
This matches what "git foo --help" does so it's widely accepted.

Keep around a fallback for a while, in case users/packagers fail to
install the new "man" dependency.

In future, "abbr -h" (as opposed to "abbr --help") could show a more
concise version, not sure.

Closes #11786

debian/control

@@ -25,8 +25,8 @@ Depends: bsdextrautils | bsdmainutils,
  file,
 # for the msgfmt command
  gettext-base,
-# for nroff and preconv
- groff-base,
+# for man
+ man-db,
 # for terminal definitions
  ncurses-base,
 # for kill