Network device has to be available when network stack is created, but
sometimes network device creation should wait for device initialization
by another cpu. This patch makes it possible to delay network stack
creation until network device is available.
POSIX stack does not allow one to bind more than one socket to given
port. Native stack on the other hand does. The way services are set up
depends on that. For instance, on native stack one might want to start
the service on all cores, but on POSIX stack only on one of them.
NOTE: This patch makes pollable_fd::get_file_desc public in order to
access it in posix_server_socket_impl::accept(). Maybe there is a better
solution. But, fow now, this avoids a lot of including game.
