gpgsm: Create binary detached sigs with definite form length octets.

* sm/sign.c: Include tlv.h.
(write_detached_signature): New,
(gpgsm_sign): Fixup binary detached signatures.

This helps some other software to verify detached signatures.

(cherry picked from commit 8996b0b655952fa6b5bb678a92d3106f72f80f2a)