merge revision(s) d6a2bce6,c687be4b: [Backport #16159]

	time.c (find_time_t): fix round-to-zero bug

	`find_time_t` did not work correctly for year older than the Epoch
	because it used C's integer division (which rounds negative to zero).

	For example, `TIme.new(1933)` returned a wrong time whose year is 1922
	in Asia/Kuala_Lumpur because there is no 00:00:00 1st Jan. 1933 in the
	time zone.

	```
	$ TZ=Asia/Kuala_Lumpur ruby -e 'p Time.new(1933)'
	1932-12-31 00:00:00 +0700
	```

	This change fixes the issue by using `DIV` macro instead of `/`.
	Now `Time.new(1933)` returns a time in 1933.

	```
	$ TZ=Asia/Kuala_Lumpur ruby -e 'p Time.new(1933)'
	1933-01-01 00:20:00 +0720
	```

	[Bug #16159]

	Added a test for [Bug #16159]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67864 b2dd03c8-39d4-4d8f-98ff-823fe69b080e