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

* tools/card-tool-misc.c (pubkey_algo_string): Move to  ...
* common/sexputil.c (pubkey_algo_string): here.

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.

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