Add dns_rdata_fromtext() fuzzer

Add dns_rdata_fromtext() fuzzer

... along with dns_rdataclass_fromtext and dns_rdatatype_fromtext

Most of the test binary is modified named-rrchecker. Main differences:
- reads single RR and exists
- does not refuse meta classes and rr types
We actually do have some fromtext code for meta-things so erroring out
in named-rrchecker would prevent us from testing this code.

Corpus has examples of all currently supported RR types. I did not do
any minimization.

In future use command

    diff -U0 \
	<(sed -n -e 's/^.*fromtext_\(.*\)(.*$/\1/p' lib/dns/code.h | \
		sort) \
	<(ls fuzz/dns_rdata_fromtext.in/)

to check for missing RR types.