simplify dns_name_fromtext() interface

previously, dns_name_fromtext() took both a target name and an
optional target buffer parameter, which could override the name's
dedicated buffer. this interface is unnecessarily complex.

we now have two functions, dns_name_fromtext() to convert text
into a dns_name that has a dedicated buffer, and dns_name_wirefromtext()
to convert text into uncompressed DNS wire format and append it to a
target buffer.

in cases where it really is necessary to have both, we can use
dns_name_fromtext() to load the dns_name, then dns_name_towire()
to append the wire format to the target buffer.

lib/dns/peer.c

@@ -506,7 +506,7 @@ dns_peer_setkeybycharp(dns_peer_t *peer, const char *keyval) {
 	isc_buffer_constinit(&b, keyval, strlen(keyval));
 	isc_buffer_add(&b, strlen(keyval));
 	result = dns_name_fromtext(dns_fixedname_name(&fname), &b, dns_rootname,
-				   0, NULL);
+				   0);
 	if (result != ISC_R_SUCCESS) {
 		return result;
 	}