Add config_error_nonbool() helper function

Add config_error_nonbool() helper function

This is used to report misconfigured configuration file that does not
give any value to a non-boolean variable, e.g.

	[section]
		var

It is perfectly fine to say it if the section.var is a boolean (it means
true), but if a variable expects a string value it should be flagged as
a configuration error.

Signed-off-by: Junio C Hamano <gitster@pobox.com>