Disable warning when reading CSRs from stdin.

Coming across the same warning that was reported in
[PR#929](dehydrated-io#929 "Suppress
openssl warning about reading from stdin") this is my attempt to disable
this warning. Instead of discarding stderr in total (this can still be
useful), we just use the "-in" parameter as hinted in the warning:

 $ foo=$(cat req.csr)
 $ <<<${foo} openssl req -noout -verify > /dev/null; echo $?
 Warning: Will read cert request from stdin since no -in option is given
 0

 $ <<<${foo} openssl req -in - -noout -verify > /dev/null; echo $?
 0

dehydrated

@@ -1011,11 +1011,11 @@ signed_request() {
 extract_altnames() {
   csr="${1}" # the CSR itself (not a file)
 
-  if ! <<<"${csr}" "${OPENSSL}" req -verify -noout >/dev/null 2>&1; then
+  if ! <<<"${csr}" "${OPENSSL}" req -in - -verify -noout >/dev/null; then
     _exiterr "Certificate signing request isn't valid"
   fi
 
-  reqtext="$( <<<"${csr}" "${OPENSSL}" req -noout -text )"
+  reqtext="$( <<<"${csr}" "${OPENSSL}" req -in - -noout -text )"
   if <<<"${reqtext}" grep -q '^[[:space:]]*X509v3 Subject Alternative Name:[[:space:]]*$'; then
     # SANs used, extract these
     altnames="$( <<<"${reqtext}" awk '/X509v3 Subject Alternative Name:/{print;getline;print;}' | tail -n1 )"
@@ -1268,7 +1268,7 @@ sign_csr() {
 
   # Finally request certificate from the acme-server and store it in cert-${timestamp}.pem and link from cert.pem
   echo " + Requesting certificate..."
-  csr64="$( <<<"${csr}" "${OPENSSL}" req -config "${OPENSSL_CNF}" -outform DER | urlbase64)"
+  csr64="$( <<<"${csr}" "${OPENSSL}" req -in - -config "${OPENSSL_CNF}" -outform DER | urlbase64)"
   if [[ ${API} -eq 1 ]]; then
     crt64="$(signed_request "${CA_NEW_CERT}" '{"resource": "new-cert", "csr": "'"${csr64}"'"}' | "${OPENSSL}" base64 -e)"
     crt="$( printf -- '-----BEGIN CERTIFICATE-----\n%s\n-----END CERTIFICATE-----\n' "${crt64}" )"