Fix extraction of week and quarter fields from intervals.

Fix extraction of week and quarter fields from intervals.

"EXTRACT(WEEK FROM interval_value)" formerly threw an error.
Define it as "tm->tm_mday / 7".  (With C99 division semantics,
this gives consistent results for negative intervals.)

"EXTRACT(QUARTER FROM interval_value)" has been implemented
all along, but it formerly gave extremely strange results for
negative intervals.  Fix it so that the output for -N months
is the negative of the output for N months.

Per bug #18348 from Michael Bondarenko and subsequent discussion.

Discussion: https://postgr.es/m/18348-b097a3587dfde8a4@postgresql.org