ALTER TYPE 4: temporal data types

Add exemptor functions to avoid rewrites for conversions involving the temporal
data types.  I needed a find-last-set function for the interval_scale exemptor
function, so I imported one from FreeBSD.  To improve timestamp<->timestamptz
when the timezone "is" UTC/GMT, I compare the current timezone definition to the
gmt_timezone we keep handy.  To make that actually work, I've changed pg_tzset
to zero its unused bytes.