common: Add an easy to use DER builder.

* common/tlv-builder.c: New.
* common/tlv.c: Remove stuff only used by GnuPG 1.
(put_tlv_to_membuf, get_tlv_length): Move to ...
* common/tlv-builder.c: here.
* common/tlv.h (tlv_builder_t): New.

Such code should actually go into libksba and we will eventually do
that. However, for now it is easier to keep it here.

• Signed-off-by: Werner Koch <wk@gnupg.org>