net: sockets: move select() implementation to zvfs

Move the implementation of zsock_select() to zvfs_select(). This
allows other types of file descriptors to also make use of
select() functionality even when the network subsystem is not
enabled.

Additionally, it partially removes a dependency cycle between
posix and networking by moving functionality into a mutual
dependency.

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