nm: Fix password entry for private keys and allow saving it

nm: Fix password entry for private keys and allow saving it

On newer desktops the auth dialog is called with --external-ui-mode and
it seems that the password flag has to be set, otherwise the password is
not stored temporarily in the profile and passed to charon-nm (not sure
how this works exactly as need_secrets() is called multiple times even
after the password was already entered, only before doing so the last
time is the password available in that callback, but only if the flag
was set).  This now also allows storing the password for the private key
with the profile.

Fixes #3428.