Add --cert option to allow self-certified https: JSON-RPC servers

When using distributed miners to a central coin server, the RPC username
and password travel unencrypted over the net, allowing any snooper to empty
your wallet.  To defeat this, we would use https: but OpenSSL fails if the
certificate is self-signed, which is the normal case for weebs like me.

This patch adds a --cert option allowing users to supply the RPC server's
self-signed certificate.

The same modification has already been accepted by pooler/cpuminer
https://github.com/pooler/cpuminer/commit/805ea94a4d2701c99340e90501bac300b9aca63e
but here it has been adapted to the multiple-pool structure of cgminer, though tested only with a single pool.