add CURLOPT_PROXY_ISSUERCERT for consistency

[gvollant]

CURLOPT_ISSUERCERT exists but not CURLOPT_PROXY_ISSUERCERT

the openssl vtls is already compatible with issuercert against proxy

[bagder]

This commit doesn't add any code that uses these options, is that already in place?

[gvollant]

in openssl.c we have already

if(SSL_SET_OPTION(issuercert) || SSL_SET_OPTION(issuercert_blob)) {

and SSL_SET_OPTION will uses proxy parameters if needed

[bagder]

Thanks!