BUG/MINOR: ssl: Encrypted keys could not be loaded when given alongside certificate

BUG/MINOR: ssl: Encrypted keys could not be loaded when given alongside certificate

The SSL passphrase callback function was only called when loading
private keys from a dedicated file (separate from the corresponding
certificate) but not when both the certificate and the key were in the
same file.
We can now load them properly, regardless of how they are provided.
A flas had to be added in the 'passphrase_cb_data' structure because in
the 'ssl_sock_load_pem_into_ckch' function, when calling
'PEM_read_bio_PrivateKey' there might be no private key in the PEM file
which would mean that the callback never gets called (and cannot set the
'passphrase_idx' to -1).

This patch can be backported to 3.3.