apps x509: restrict CAkeyform option to OPT_FMT_PDE

CAkeyform may be set to PEM, DER or ENGINE, but the current options
are not using the proper optionformat 'E' (OPT_FMT_PDE) for this.

Set the valtype for CAkeyform to 'E' and use OPT_FMT_PDE when extracting
the option value.

This amends bf4006a6 ("Fix regression on x509 keyform argument") which
did the same thing for keyform and changed the manpage synopsis entries
for both keyform and CAkeyform but did not change the option section.
Hence, change the option section.

CLA: trivial

Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11172)