nss: make FTPS work with --proxytunnel

If the NSS code was in the middle of a non-blocking handshake and it
was asked to finish the handshake in blocking mode, it unexpectedly
continued in the non-blocking mode, which caused a FTPS connection
over CONNECT to fail with "(81) Socket not ready for send/recv".

Bug: https://bugzilla.redhat.com/1420327