acme-client: simplify elliptic curve signatures

acme-client: simplify elliptic curve signatures

We can get the correct size of the signature using EVP_PKEY_bits() which
uses the order instead of the (strictly speaking incorrect) degree. Grab
the (r, s) out of the ECDSA signature with ECDSA_SIG_get0_{r,s}(), which
is a saner interface than EVP_SIG_get0(). Finally, do the zero padding
using BN_bn2binpad() which is simpler than the currently rather fiddly
solution.

ok jsing