SimpleLink net_if impl w/ async support; MG_F_SSL

SimpleLink sockets are suffciently different from BSD that all the
ifdefs have become too messy to warrant a separate net_if
implementation. As part of this we also implement proper async connect
support.

Added MG_F_SSL to identify SSL-enabled connections in a generic way,
since SSL state can be different depending on the implementation.

PUBLISHED_FROM=9cdb8c880b90683e4a26b972cf439d47d6f60917