CCBC-246 Fallback to 'select' if default plugin cannot be loaded

On UNIX-like systems, default IO backend is libevent, which is
third-party library and the plugin itself goes in separate package and
could be not available at the run-time

This patch allows to fallback to 'select' plugin only if the user will
pick default IO backend

Change-Id: I834b35f5614d10da75635899589192b073a2e91b
Reviewed-on: http://review.couchbase.org/28374
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>