common: New function get_keyalgo_string.

* common/openpgp-oid.c (struct keyalgo_string_s): New.
(keyalgo_strings): New.
(keyalgo_strings_size, keyalgo_strings_used): New.
(openpgp_oid_or_name_to_curve): New.
(get_keyalgo_string): New.
--

This function is intended as a more general version of gpg's
pubkey_string function.  It has the advantage to avoid mallocs and
uses static table of algorithm strings instead.  There should be only
a few dozen of such strings (if at all) and thus all those allocations
we do internally in gpg's pubkey_string and the static buffers all
over the place are not too nice.

Signed-off-by: Werner Koch <wk@gnupg.org>
(cherry picked from commit 3a1fa13eed)
(cherry picked from commit 332a72f734)

g10/keyid.c

@@ -88,7 +88,10 @@ pubkey_letter( int algo )
    "256E"  - ECDSA using a curve with 256 bit
 
    The macro PUBKEY_STRING_SIZE may be used to allocate a buffer with
-   a suitable size.*/
+   a suitable size.  Note that a more general version of this function
+   exists as get_keyalgo_string.  However, that has no special
+   treatment for the old and unsupported Elgamal which we here print as
+   xxxNNNN.  */
 char *
 pubkey_string (PKT_public_key *pk, char *buffer, size_t bufsize)
 {