Implement commands for management of resident keys

Implement command 0x41 which is used by OpenSSH for reading RKs. It has
the following subcommands:
 * CMD_CRED_METADATA - get number of saved/remaining RKs
 * CMD_RP_BEGIN/CMD_RP_NEXT - iterate over the saved RPs
 * CMD_RK_BEGIN/CMD_RK_NEXT - iterate over the RKs for a given RP

Fixes issue #374 and issue #314

fido2/log.c

@@ -51,6 +51,7 @@ struct logtag tagtable[] = {
     {TAG_NFC,"NFC"},
     {TAG_NFC_APDU, "NAPDU"},
     {TAG_CCID, "CCID"},
+    {TAG_CM, "CRED_MGMT"},
 };