utility: Detect GNU 'ls' before applying the relevant tweaks

Detect `ls` provided by GNU Core Utilities upfront before applying
the relevant settings and aliases.

The revised logic takes the following aspects into consideration:
- `is-callable 'dircolors'` is not a sufficient test for verifying if
  the `ls` in scope is _also_ provided by GNU Core Utilities. `ls` can
  continue to be the one provided by BSD Core Utilities because of its
  precedence in `$path`.
- The `ls` in scope anyway can be either GNU Utils provided or BSD Utils
  provided depending on either `$path` precedence, soft link (via
  `ln -s`), or wrapper function (via 'gnu-utility' module).
- So instead of '_guessing_' the possible one, just detect if it is from
  GNU Utils and apply the relevant settings and aliases.
- Note that GNU prefixed `ls` (`gls`) is intentionally not honored in
  this case to avoid possible conflict/shadowing with other `gls`
  callable (can happen if 'git' module is used, for example).
  Besides, honoring `gls` would imply we honor other commands (like
  `mv`, `rm` etc.) to be consistent. We have a module 'gnu-utils'
  dedicated for that after all.

modules/utility/README.md

@@ -2,6 +2,11 @@
 
 Defines general aliases and functions.
 
+**Note:** Some of the utilities configured in this module might be provided via
+GNU utilities with incompatible arguments on non-GNU systems. In such cases,
+using [*`gnu-utility`*][1] module is recommended and it must be loaded
+**before** this module.
+
 ## Settings
 
 ### Highlighting
@@ -107,7 +112,7 @@ zstyle ':prezto:module:utility' correct 'no'
 - `lt` lists sorted by date, most recent last.
 - `lc` lists sorted by date, most recent last, shows change time.
 - `lu` lists sorted by date, most recent last, shows access time.
-- `sl` lists directory contents (`ls`).
+- `sl` lists directory contents (correction for `ls`).
 
 ### macOS Everywhere
 
@@ -181,10 +186,11 @@ In addition, the following aliases have been added:
 
 ## Authors
 
-*The authors of this module should be contacted via the [issue tracker][1].*
+*The authors of this module should be contacted via the [issue tracker][2].*
 
 - [Robby Russell](https://github.com/robbyrussell)
 - [Suraj N. Kurapati](https://github.com/sunaku)
 - [Sorin Ionescu](https://github.com/sorin-ionescu)
 
-[1]: https://github.com/sorin-ionescu/prezto/issues
+[1]: ../gnu-utility#readme
+[2]: https://github.com/sorin-ionescu/prezto/issues