dns_regru: require a dot boundary in root zone matching

_get_root matched a registered domain anywhere as a substring of the
challenge domain, so with both "test.com.ru" and "subtest.com.ru" in the
account, issuing for subtest.com.ru wrongly picked test.com.ru as the
root (it is a substring of "sub-test.com.ru"). Anchor the match to a '.'
boundary so a shorter domain no longer matches a longer subdomain label.

Fixes the issue reported in #5036 (thanks @koledas)
Closes #5036

dnsapi/dns_regru.sh

@@ -96,8 +96,8 @@ _get_root() {
 
   for ITEM in ${domains_list}; do
     IDN_ITEM=${ITEM}
-    case "${domain}" in
-    *${IDN_ITEM}*)
+    case ".${domain}" in
+    *.${IDN_ITEM}*)
       _domain="$(_idn "${ITEM}")"
       _debug _domain "${_domain}"
       return 0