gpgsm: New compatibility flag "allow-ecc-encr".

* sm/gpgsm.h (COMPAT_ALLOW_ECC_ENCR): New.
* sm/gpgsm.c (compatibility_flags): Add new flag.
* sm/encrypt.c (encrypt_dek): Allw ECC only if flag is set.

ECC encryption was not part of the original VS evaluation. Until this
has been re-evaluated we hide this feature behind this flag.

• GnuPG-bug-id: T6253