Remove the list of key algorithms in pageant.c.

Remove the list of key algorithms in pageant.c.

The only reason those couldn't be replaced with a call to the
centralised find_pubkey_alg is because that function takes a zero-
terminated string and instead we had a (length,pointer) string. Easily
fixed; there's now a find_pubkey_alg_len(), and we call that.

This also fixes a string-matching bug in which the sense of memcmp was
reversed by mistake for ECDSA keys!