Both of the defaults we currently have in the curl driver are named
based on a slightly different schema, let's unify that and call both
CURL_BLOCK_OPT_${NAME}_DEFAULT.

While at it, we can add a macro for the third option for which a default
exists, namely "sslverify".

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190201192935.18394-28-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>