Fix auto-discovery of ENGINEs, ported from HEAD.

NB, this fixes a regression relative to 0.9.7 and the documented behaviour,
but it would make sense for distro maintainers and others with an interest
in system behaviour to test with this change. The fix re-enables behaviour
that was broken and thus inherently disabled. In particular, if you
register an ENGINE implementation, and that ENGINE is able to successfully
self-initialise on the host, it will get used automatically (as claimed in
the documentation and as was the case for 0.9.7) - this was not the case
with 0.9.8 until now because of a bug.

PR: 1668
Submitted by: Ian Lister
Reviewed by: Geoff Thorpe