bug#66279: Unexporting <user-account>

[Ludovic Courtès]

Hi Maxim,

Commit 03795e2ba27424fc98957da00f6c71325e7ae425 exports the
<user-account> record type descriptor (RTD).

Common practice is to keep RTDs private because by publishing them, we
make it harder to change the ABI (because users might be matching fields
positionally) and we make it trivial for users to forge records of that
type, bypassing any checks we may have in the official constructor (such
as “sanitizers”).

What do you think of reverting this commit?  I don’t see references to
<user-account> outside of its module.

(I’m aware there are a few other places where RTDs are exported; I think
we should eventually fix them as well.)

Ludo’.