Stringify many completions and functions, with --invert stringification.

I believe apm must have been buggy - example output that I found online
showed `tr` was mangling paths with spaces in it. Should be fixed.

Also, use dscl on OS X in __fish_complete_users.fish like
__fish_print_users.fish already does.

share/tools/web_config/sample_prompts/acidhub.fish

@@ -12,7 +12,7 @@ function fish_prompt -d "Write out the prompt"
     if [ (_git_branch_name) ]
         set -l git_branch (set_color -o blue)(_git_branch_name)
         if [ (_is_git_dirty) ]
-            for i in (git branch -qv --no-color|grep \*|cut -d' ' -f4-|cut -d] -f1|tr , \n)\
+            for i in (git branch -qv --no-color| string match -r \*|cut -d' ' -f4-|cut -d] -f1|tr , \n)\
                 (git status --porcelain | cut -c 1-2 | uniq)
                 switch $i
                     case "*[ahead *"

share/tools/web_config/sample_prompts/nim.fish

@@ -3,7 +3,7 @@
 
 function fish_prompt
     and set retc green; or set retc red
-    tty|grep -q tty; and set tty tty; or set tty pts
+    tty|string match -q -r tty; and set tty tty; or set tty pts
 
     set_color $retc
     if [ $tty = tty ]
@@ -48,7 +48,7 @@ function fish_prompt
     echo -n ]
     
     if type -q acpi
-		if [ (acpi -a 2> /dev/null | grep off) ]
+		if [ (acpi -a 2> /dev/null | string match -r off) ]
 			echo -n '─['
 			set_color -o red
 			echo -n (acpi -b|cut -d' ' -f 4-)