net: sockets: implement shutdown() method for net_context sockets

Add basic shutdown() implementation for net_context sockets, which
handles only SHUT_RD as 'how' parameter and returns -ENOTSUP for SHUT_WR
and SHUT_RDWR. The main use case to cover is to allow race-free wakeup
of threads calling recv() on the same socket.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>