net: sockets: move poll implementation to zvfs

Move the implementation of zsock_poll to zvfs_poll. This allows
other types of file descriptors to also make use of poll()
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>