Polish up the PuTTYgen user interface for ECC key types.

Jacob pointed out that a free-text field for entering a key size in
bits is all very well for key types where we actually _can_ generate a
key to a size of your choice, but less useful for key types where
there are only three (or one) legal values for the field, especially
if we don't _say_ what they are.

So I've revamped the UI a bit: now, in ECDSA mode, you get a dropdown
list selector showing the available elliptic curves (and they're even
named, rather than just given by bit count), and in ED25519 mode even
that disappears. The curve selector for ECDSA and the bits selector
for RSA/DSA are independent controls, so each one remembers its last
known value even while temporarily hidden in favour of the other.

The actual generation function still expects a bit count rather than
an actual curve or algorithm ID, so the easiest way to actually
arrange to populate the drop-down list was to have an array of bit
counts exposed by sshecc.c. That's a bit ugly, but there we go.

One small functional change: if you enter an absurdly low value into
the RSA/DSA bit count box (under 256), PuTTYgen used to give a warning
and reset it to 256. Now it resets it to the default key length of
2048, basically because I was touching that code anyway to change a
variable name and just couldn't bring myself to leave it in a state
where it intentionally chose such an utterly useless key size. Of
course this doesn't prevent generation of 256-bit keys if someone
still really wants one - it just means they don't get one selected as
the result of a typo.

ssh.h

@@ -155,6 +155,7 @@ struct ec_curve {
 const struct ssh_signkey *ec_alg_by_oid(int len, const void *oid,
                                         const struct ec_curve **curve);
 const unsigned char *ec_alg_oid(const struct ssh_signkey *alg, int *oidlen);
+extern const int ec_nist_curve_lengths[], n_ec_nist_curve_lengths;
 const int ec_nist_alg_and_curve_by_bits(int bits,
                                         const struct ec_curve **curve,
                                         const struct ssh_signkey **alg);