Fix mess up of internal/external paths (from Enrico Forestieri)

Fix mess up of internal/external paths (from Enrico Forestieri)
	* src/frontends/qt2/QPrefs.C:
	(internal_path_list, external_path_list): Handle the PATH prefix
	style according to the user-prefs style.
	(QPrefs::apply, QPrefs::update_contents): fix inverted logic of
	rc.cygwin_path_fix

	* src/support/os.h: New declarations to deal with PATH lists.
	* src/support/os_cygwin.C: Major rewrite to account for path style
	problems.
	* src/support/os_unix.C:
	* src/support/os_win32.C: Stub functions for PATH lists.
	* src/support/environment.C: fix bug 2344: Wrong path_prefix handling
	in cygwin
	* src/support/filetools.C (ChangeExtension): fix for the path
	style to be written in .tex files (cygwin related).

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@13558 a592a061-630c-0410-9148-cb99ea01b6c8