Fix apropos completions

The 'str' variable was apparently mistakenly removed by 49c5f96470.

Re-add it, and regex-escape it as well.

Allow completing on apropos <TAB> instaed of requiring an initial char.

Use __fish_apropos instead of apropos.

New regex to hopefully work on more platforms.

Explicitly use ^ instead of adding it at __fish_apropos

share/functions/__fish_apropos.fish

@@ -36,7 +36,7 @@ if test $status -eq 0 -a (count $sysver) -eq 3
             set age (path mtime -R -- $whatis)
         end
 
-        MANPATH="$dir" apropos "^$argv"
+        MANPATH="$dir" apropos "$argv"
 
         if test $age -ge $max_age
             test -d "$dir" || mkdir -m 700 -p $dir
@@ -46,6 +46,8 @@ if test $status -eq 0 -a (count $sysver) -eq 3
     end
 else
     function __fish_apropos
-        apropos $argv
+        # we only ever prefix match for completions. This also ensures results for bare apropos <TAB>
+        # (apropos '' gives no results, but apropos '^' lists all manpages)
+        apropos "$argv"
     end
 end