Unify winsock error handling

This provides a unified switch statement for mapping the correct winsock
error code to the equivalent 'standard' one. Rather than having this
fragmented in various places, we've moved the mappings to the io plugin
helper directory (<libcouchbase/plugins/io)

Change-Id: Iad54ec7f837fe9c1d43211c10642eff9d837c81f
Reviewed-on: http://review.couchbase.org/36088
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>