DEV-35248: Allow NoVerifySSLCert to uniformly disable verification

Introduce static LLCurl SSL verification flag, default 'true', accessed by
LLCurl::setSSLVerify() and getSSLVerify().
Make LLCurl::Easy::prepRequest() check LLCurl::getSSLVerify() instead of
unconditionally setting CURLOPT_SSL_VERIFYPEER 'true'. Also set
CURLOPT_SSL_VERIFYHOST to match.
Make LLXMLRPCTransaction::Impl::init() examine LLCurl::getSSLVerify(), instead
of directly examining gSavedSettings.getBOOL("NoVerifySSLCert").
Make LLURLRequest::checkRootCertificate() set CURLOPT_SSL_VERIFYHOST as well
as CURLOPT_SSL_VERIFYPEER.
Make request() in llhttpclient.cpp (used by LLHTTPClient::getByteRange(),
head(), get(), getHeaderOnly(), put(), post(), postRaw(), postFile(), del(),
move()) pass LLCurl::getSSLVerify() to checkRootCertificate(), rather than
constant 'true'.
Make LLAppViewer::mainLoop() call
    LLCurl::setSSLVerify(! gSavedSettings.getBOOL("NoVerifySSLCert"))
at the same time it calls LLCurl::setCAFile(), a comparable bit of static
setup.