net: socket: additional POSIX constants

The POSIX spec requires that `SO_LINGER`, `SO_RCVLOWAT`,
and `SO_SNDLOWAT`, and `SOMAXCONN` are defined in
`<sys/socket.h>`. However, most of the existing socket
options and related constants are defined in
`<zephyr/net/socket.h>`.

For now, we'll co-locate them. It would be
good to properly namespace things.

Additionally, a no-op for setsockopt for `SO_LINGER` to
make things Just Work (TM) for now.

Signed-off-by: Chris Friedt <cfriedt@meta.com>