Make argparse reject supplying a validator for boolean flags

Specifically, this commit simply makes argparse issue an error if you use the !
syntax to define a validation script on an option that does not take any
arguments. For example, "argparse foo!exit -- --foo" is now an error. This was
previously accepted, despite that fact that the code after ! would never be
executed (the ! code is only executed when an option is given a value).

Alternatively, ! validation scripts could be made to execute even when no value
was provided, but this break existing code that uses them with flags that take
optional values.

doc_src/cmds/argparse.rst

@@ -156,7 +156,9 @@ Each option specification consists of:
 
 - Optionally a ``&``, indicating that the option and any attached values are not to be saved in ``$argv`` or ``$argv_opts``. This does not affect the the ``_flag_`` variables.
 
-- Optionally a ``!`` followed by fish script to validate the value. Typically this will be a function to run. If the exit status is zero the value for the flag is valid. If non-zero the value is invalid. Any error messages should be written to stdout (not stderr). See the section on :ref:`Flag Value Validation <flag-value-validation>` for more information.
+- Nothing if the flag is a boolean that takes no argument, or
+
+    - ``!`` followed by fish script to validate the value. Typically this will be a function to run. If the exit status is zero the value for the flag is valid. If non-zero the value is invalid. Any error messages should be written to stdout (not stderr). See the section on :ref:`Flag Value Validation <flag-value-validation>` for more information.
 
 See the :doc:`fish_opt <fish_opt>` command for a friendlier but more verbose way to create option specifications.