Use basic auth with ldap-dn and ldap-password in
puavo-sync-external-files.
Use PUAVO_ETC to lookup ldap_dn and ldap_password.
This is even cleaner way to do basic auth in
puavo-sync-external-files, but still it is wrong
because it breaks if password contains '/'-characters.
puavo-sync-external-files: Implement get-requests
a bit differently.  This allows using strange characters in
username and password.
Do not use standard error for messages, our messages are not
errors but simple normal operation.