cppinit.c (remove_dup_dir): Add head_ptr argument to handle removal at head.

	* cppinit.c (remove_dup_dir): Add head_ptr argument to handle removal
	at head.
	(remove_dup_nonsys_dirs): New function.
	(remove_dup_dirs): Change argument head to head_ptr.  Remove warnings.
	(merge_include_chains): Remove non-system include directories from
	quote and bracket include chains when they duplicate equivalent system
	directories.
	* doc/cpp.texi (-I): Update.
        * doc/cppopts.texi (-I): Update.
        * doc/install.texi (--with-local-prefix): Further document usage of
	this option.
	* doc/invoke.texi (-I): Update.

From-SVN: r56468