sockets: use positive flag for file descriptor socket reference

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().

Reviewed by: tuexen
Differential revision: https://reviews.freebsd.org/D35678