common: Extend openpgp_oid_to_curve to return an abbreviated name.

* common/openpgp-oid.c (oidtable): Add column "abbr" and set them for
Brainpool.
(openpgp_oid_to_curve): Rename arg "canon" to "mode" and implement
mode 2.

For dual algorithms (PQC) we need shorter versions of brainpool to
avoid names which otherwise might be capped when printed.