Modify the dkeyform type to support engine

Modify the dkeyform type to support engine

The valtype value of dkeyform defined in the s_server_options structure is F, which leads to the judgment that the engine is not supported when processing parameters in the opt_next function.
This the valtype value of dkeyform should be changed to "f".

CLA: trivial

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21982)