Add strings.h and unistd.h wrappers for win32

Change-Id: I9dab3653fb4e53eb3b669077f116b0ccb8348a51
Reviewed-on: http://review.couchbase.org/32416
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>