SSL: Avoid magic allocation of SSL backend specific data

Originally, my idea was to allocate the two structures (or more
precisely, the connectdata structure and the four SSL backend-specific
strucutres required for ssl[0..1] and proxy_ssl[0..1]) in one go, so
that they all could be free()d together.

However, getting the alignment right is tricky. Too tricky.

So let's just bite the bullet and allocate the SSL backend-specific
data separately.

As a consequence, we now have to be very careful to release the memory
allocated for the SSL backend-specific data whenever we release any
connectdata.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Closes #2119