Support Swift 5

ci: Update to Swift 4.2.3, use official Docker images

When attempting to connect a socket that is already connected, the error code being returned was correct but the error message text that was being returned was the exact opposite of the actual error.

Minor code change to use the nil coalescing to improve readability.  Improved internal documentation to call out specific notations supported.

Added new parameter to TCP version of listen API to allow caller to specify a particular address to listen on.  This new parameter is defaulted to nil and therefore should affect current callers.