Result : fails.
Reason : loopback passphrase provider is never called.
With default pinentry mode, the passphrase is requested normally
and the private key is exported. But this can't be done on a web server.
Enclosed in #ifdef DEVTIME.
In-place editing with a popup if user is allowed in app config file.
User must of course manage at least a private key. User identities of
target key may be selectively chosen. Optionally, certification may be
exportable and non-revocable.