socket: Remove NOFREE from the socket zone

This flag was added during the transition away from the legacy zone
allocator, commit c897b81311792ccf6a93feff2a405e2ae53f664e. The old
zone allocator effectively provided _NOFREE semantics, but it seems that
they are not required for sockets. In particular, we use reference
counting to keep sockets live.

One somewhat dangerous case is sonewconn(), which returns a pointer to a
socket with reference count 0. This socket is still effectively owned
by the listening socket. Protocols must therefore be careful to
synchronize sonewconn() calls with their pru_close implementations,
since for listening sockets soclose() will abort the child sockets. For
example, TCP holds the listening socket's PCB read locked across the
sonewconn() call, which blocks tcp_usr_close(), and sofree()
synchronizes with a concurrent soabort() of the nascent socket.
However, _NOFREE semantics are not required here.

Eliminating _NOFREE has several benefits: it enables use-after-free
detection (e.g., by KASAN) and lets the system reclaim memory from the
socket zone under memory pressure. No functional change intended.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31975