SSL.Cipher: Added support for ECDSA certificates.

This implements low-level support for RFC 4492 (TLSECC) suites.
SSL: Added quite a few ECDSA suites.
Testsuite: Test SSL with ECDSA.
SSL.https: Support ECDSA_MODE.
[ia32] At least gentoo provides a REG_* enum, in sys/ucontext.h

Thus, move to using a more unique naming scheme - P_REG_* etc.
bignum: always define unsigned sub overflow check

The unsigned sub overflow check was not defined on x86_32, but
referenced.
SSL: Support Nettle without ECC again.