Adds support for reading TSIG text format.

Adds support for reading TSIG text format.

Implements from_text for the TSIG record type, and clean up some other
things.

Fixes the text format to emit fields in the right order; fudge and
time_signed were reversed.  This also matches BIND's output format now.

Add get_uint48() to the tokenizer, so that from_text() can use it.  Add
get_uint48() to the wire parser, and use it in from_wire, for
consistency.

Change dns.tsig.sign() to use rdata.replace() rather than constructing a
new TSIG record manually; this couldn't be done before, because
replace() uses text format for validation.