Fix some minor time-related bugs with POSIX time arguments.

	Fix some minor time-related bugs with POSIX time arguments.
	Some valid time stamps were being rejected (notably -1, and
	time stamps before 1900 on 64-bit hosts).  And some invalid
	time stamps were being accepted, e.g. September 31.

	* posixtm.h (posixtime): Return bool instead of time_t, so
	that we can return (time_t) -1 successfully.
	* posixtm.c: Likewise.
	[HAVE_STDBOOL_H]: Include <stdbool.h>.
	(bool, false, true) [!HAVE_STDBOOL_H]: New type.
	(t): Remove static var.
	(year, posix_time_parse): Now takes struct tm * arg to modify, instead
	of static var.  All uses changed.
	(year): Do not reject years before 1900; they can occur with
	64-bit time_t.
	(posix_time_parse): Do not check for out-of-range components;
	that is now the caller's responsibility, since our checks were
	only approximations.
	(posixtime): Use mktime to check for out-of-range components,
	since it knows them exactly.
	If mktime returns (time_t) -1, check whether an error actually occurred
	by invoking localtime on -1.
	(main) [TEST_POSIXTIME]: Check for input data errors, and report
	posixtime failures better.
	Improve the test data (in comments only).