test-server: fix to compile using mingw

Signed-off-by: Alejandro Mery <amery@geeks.cl>
context: unify libwebsocket_create_context()'s return

Signed-off-by: Alejandro Mery <amery@geeks.cl>
context: move libev specifics into a sub-struct

Signed-off-by: Alejandro Mery <amery@geeks.cl>
context: move poll/win32 specifics into a sub-struct

Signed-off-by: Alejandro Mery <amery@geeks.cl>
introduce info->event and info->e to select and configure a backend

Signed-off-by: Alejandro Mery <amery@geeks.cl>
introduce context->event_ops

Signed-off-by: Alejandro Mery <amery@geeks.cl>
libev: move loop setting and backend printing to event_ops->init()

Signed-off-by: Alejandro Mery <amery@geeks.cl>