Use find_pubkey_alg in openssh_read_new().

This is better than listing all the algorithm names in yet another
place that will then need updating when a new key format is added.
However, that also means I need to find a new place to put the
'npieces' value I was previously setting up differently per key type;
since that's a fundamental property of the key format, I've moved it
to a constant field in the ssh_signkey structure, and filled that
field in for all the existing key types with the values from the
replaced code in openssh_read_new().

sshdss.c

@@ -701,6 +701,7 @@ const struct ssh_signkey ssh_dss = {
     dss_createkey,
     dss_openssh_createkey,
     dss_openssh_fmtkey,
+    5 /* p,q,g,y,x */,
     dss_pubkey_bits,
     dss_fingerprint,
     dss_verifysig,