Rename SS_NOFDREF to SS_FDREF and flip all bitwise operations.
Mark sockets created by socreate() with SS_FDREF.
This change is mostly illustrative. With it we see that SS_FDREF
is a debugging flag, since:
- socreate() takes a reference with soref().
- on accept path solisten_dequeue() takes a reference with soref() and then soaccept() sets SS_FDREF.
- soclose() checks SS_FDREF, removes it and does sorele().