doh: add options to disable ssl verification

- New libcurl options CURLOPT_DOH_SSL_VERIFYHOST,
  CURLOPT_DOH_SSL_VERIFYPEER and CURLOPT_DOH_SSL_VERIFYSTATUS do the
  same as their respective counterparts.

- New curl tool options --doh-insecure and --doh-cert-status do the same
  as their respective counterparts.

Prior to this change DOH SSL certificate verification settings for
verifyhost and verifypeer were supposed to be inherited respectively
from CURLOPT_SSL_VERIFYHOST and CURLOPT_SSL_VERIFYPEER, but due to a bug
were not. As a result DOH verification remained at the default, ie
enabled, and it was not possible to disable. This commit changes
behavior so that the DOH verification settings are independent and not
inherited.

Ref: https://github.com/curl/curl/pull/4579#issuecomment-554723676

Fixes https://github.com/curl/curl/issues/4578
Closes https://github.com/curl/curl/pull/6597

src/tool_help.c

@@ -149,7 +149,7 @@ static const struct helptxt helptext[] = {
    "Client certificate file and password",
    CURLHELP_TLS},
   {"    --cert-status",
-   "Verify the status of the server certificate",
+   "Verify the status of the server cert via OCSP-staple",
    CURLHELP_TLS},
   {"    --cert-type <type>",
    "Certificate type (DER/PEM/ENG)",
@@ -241,6 +241,12 @@ static const struct helptxt helptext[] = {
   {"    --dns-servers <addresses>",
    "DNS server addrs to use",
    CURLHELP_DNS},
+  {"    --doh-cert-status",
+   "Verify the status of the DOH server cert via OCSP-staple",
+   CURLHELP_DNS | CURLHELP_TLS},
+  {"    --doh-insecure",
+   "Allow insecure DOH server connections",
+   CURLHELP_DNS | CURLHELP_TLS},
   {"    --doh-url <URL>",
    "Resolve host names over DOH",
    CURLHELP_DNS},