Add relativize_to to from_text().

Add relativize_to to from_text().

When calling from_text, the zone code needs to apply the current origin
(which may or may not be the zone origin, if sub-zone $ORIGIN statements
are present), and may also want to relativize the contents to the zone
origin.

Previously, this was done by explicitly reading records as absolute, and
then relativizing them laster.  With this change, the work is moved to
the tokenizer.

This gets rid of the remaining internal uses of
dns.rdata.choose_relativity(), which prevents rdata from being
immutable.