lwsws conf and plugins convert to libuv apis

lwsws conf and plugins convert to libuv apis

After discussion here

https://libwebsockets.org/pipermail/libwebsockets/2016-April/002268.html

scandir usage in

 - lwsws conf.c
 - lws plugin support

and

 - lws plugin apis for dl

are converted to us libuv apis so they can work cross-platform easily.

lws itself remains not requiring libuv, although it's an option.

 - LWS_WITH_LWSWS
 - LWS_WITH_PLUGINS

now force LWS_WITH_LIBUV if selected... both of these are new features
only in master atm and both are off by default in CMake.

There's a complication libuv can be too old to offer the necessary apis,
this is the case in Travis Trusty instance.  In that case, UV_VERSION_MAJOR ==0,
then the unix-only plugin implementation is used instead.

Signed-off-by: Andy Green <andy@warmcat.com>