__fish_apropos: Also add "--" separator

This actually *worked* in my tests which confuses me.

It really shouldn't, `apropos -foo` will complain about "-o" not being
a valid option.

It should be `apropos -- -foo`.

Now, of course there are awful apropos implementations, so let's see
if someone complains

share/functions/__fish_apropos.fish

@@ -30,13 +30,13 @@ function __fish_apropos
             set age (math (date +%s) - (/usr/bin/stat -f %m $db))
         end
 
-        MANPATH="$cache" apropos $argv
+        MANPATH="$cache" apropos -- $argv
 
         if test $age -ge $max_age
             mkdir -m 700 -p $cache
             /usr/libexec/makewhatis -o $db (man --path | string split :) >/dev/null 2>&1 </dev/null &
         end
     else
-        apropos $argv
+        apropos -- $argv
     end
 end