common: Provide function to get public key algo names in our format.

* common/sexputil.c (pubkey_algo_string): New.

The new gpg format for public key algorithms is useful at other places
as well. Thus we make this new function available. Note that the
code we use in gpg is not based on s-expressions and thus a new
function was required.

(cherry picked from commit 03bf8e967adb2dd13329ba1089deb419d49e55c0)

Not yet used in 2.2 but will likely be needed by future backports.

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