Hide whatis database building from the user (#8310)

* Hide whatis database building from the user

It's really an internal detail, but shows up in prompts that display how many
background jobs are running.

By disowning it keeps running but won't show up in `jobs` or get killed if the user
exits the shell.

* Update __fish_apropos.fish

share/functions/__fish_apropos.fish

@@ -44,6 +44,7 @@ if test $status -eq 0 -a (count $sysver) -eq 3
         if test $age -ge $max_age
             test -d "$dir" || mkdir -m 700 -p $dir
             /usr/libexec/makewhatis -o "$whatis" (man --path | string split :) >/dev/null 2>&1 </dev/null &
+            disown $last_pid
         end
     end
 else