SECURITY UPDATE: signature verification DoS via malicious subkey
debian/patches/CVE-2025-30258.patch: require signing usage when
looking up public key for signature verification, filtering out
subkeys without valid backsig. Include upstream regression fixes
to preserve verification of signatures from expired/revoked keys.
Widen pubkey_usage and req_usage fields from byte to u16 to
prevent PUBKEY_USAGE_VERIFY (16384) from being truncated on
GnuPG 2.2.x. Add primary-key-only lookup during import to
prevent malicious subkey attack at import time. Fix double-free
in check_signature_over_key_or_uid when signer is caller-owned.