common: New module to compute openpgp fingerprints

* common/openpgp-fpr.c: New.
* common/Makefile.am (common_sources): Add it.

This function is targeted to handle keys on smartcards.

(cherry picked from commit 2f2bdd9c0894eb43f719da8b529b4c7a46f742a0)

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