Ask for SSL key password when started with -N but without sslpassword_program.

Ask for SSL key password when started with -N but without sslpassword_program.

Do not give SSL a password-asking callback if sslpassword_program is not
configured. Without a callback, OpenSSL itself asks for the password (which
works if Squid runs in foreground because of -N).

The fix applies to Ssl::readCertChainAndPrivateKeyFromFiles() context only.
This is not the only place where we read private keys. Some other places are
working correctly, but others may need more work. Also,
Ssl::readCertChainAndPrivateKeyFromFiles() may not really work if
sslpassword_program _is_ configured because "user data" pointer will be nil.