common: Extend the openpgp_curve_to_oid function.

* common/openpgp-oid.c (openpgp_curve_to_oid): Add optional arg R_NBITS.
Change all callers.

In particular for ed25519 and cv25519 it is quite useful to have an
ability to get the required algorithm.

(cherry picked from commit 24095101a5069f15a9aea7512498ac436a76814a)