dns: treat names rejected by res_mkquery as nonexistent rather than error

this distinction only affects search, but allows search to continue
when concatenating one of the search domains onto the requested name
produces a result that's not valid. this can happen when the
concatenation is too long, or one of the search list entries is
itself not valid.

as a consequence of this change, having "." in the search domains list
will now be ignored/skipped rather than making the lookup abort with
no results (due to producing a concatenation ending in ".."). this
behavior could be changed later if needed.

src/network/lookup_name.c

@@ -153,7 +153,7 @@ static int name_from_dns(struct address buf[static MAXADDRS], char canon[static
 			qlens[nq] = __res_mkquery(0, name, 1, afrr[i].rr,
 				0, 0, 0, qbuf[nq], sizeof *qbuf);
 			if (qlens[nq] == -1)
-				return EAI_NONAME;
+				return 0;
 			qbuf[nq][3] = 0; /* don't need AD flag */
 			/* Ensure query IDs are distinct. */
 			if (nq && qbuf[nq][0] == qbuf[0][0])