* path.sgml: Remove documentation for old path API. Add documentation

	* path.sgml: Remove documentation for old path API.  Add documentation
	for new path API.
	* security.sgml: New file to document cygwin_set_impersonation_token
	and cygwin_logon_user with only eight years of delay.
	* shared.sgml: Remove file.
	* include/sys/cygwin.h (cygwin32_attach_handle_to_fd): Move declaration
	into fully deprecated function block.
	(cygwin_logon_user): Move declaration down to declaration of
	cygwin_set_impersonation_token.