Change the semantics of 'none' and 'auto' viewers/editors: 'none' means now

Change the semantics of 'none' and 'auto' viewers/editors: 'none' means now
"Don't view/edit this format", and 'auto' means "Use the default viewer/editor".
	* src/format.C
	(fixCommand): Change semantics: "none" is equal to "", and "auto"
	is removed if canAutoOpenFile returns false.

	* src/frontends/xforms/FormPreferences.C
	(FormPreferences::SpellOptions::apply): "none" -> ""

	* src/lyxrc.C
	(LyXRC::read): "none" -> ""

	* lib/doc/Customization.lyx: Document the autoopen feature

	* lib/configure.py
	(checkProg): add optional not_found argument, defaulting to 'none'
	(checkViewer): new, equivalent to checkProg with not_found = 'auto'
	(checkFormatEntries): call checkViewer instead of checkProg for
	viewers and editors

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13929 a592a061-630c-0410-9148-cb99ea01b6c8