Each implementation of "struct file" needs to implement 8 different file
operations. Most special file implementations, such as pipe, socketpair,
epoll and timerfd, don't support many of these operations. We had in
unsupported.h functions that can be reused for the unsupported operation,
but this resulted in a lot of ugly boiler-plate code.

Instead, this patch switches to a cleaner, more C++-like, method:
It defines a new "file" subclass, called "special_file", which implements
all file operations except close(), with a default implementation identical
to the old unsupported.h implementations.

The files of pipe(), socketpair(), timerfd() and epoll_create() now inherit
from special_file, and only override the file operations they really want
to implement.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>

Added Cloudius copyright statement to source files in fs/ which are our
own code (and not from Prex or FreeBSD). Also added our copyright statement
in addition to the original one when I thought we made non-trivial changes
to the original.

Sorry, missing unsupported_poll broke compilation after the previous patch

Previously, we re-implemented "unsupported" file operations (e.g., chmod
for a pipe on which fchmod makes no sense) several times - there was
an implementation only for chmod in kern_descrip.c, used in sys_socket.c,
and af_local.cc had its own. As we add more file descriptor type (e.g.,
create_epoll()) we'll have even more copies of these silly functions, so
let's do it once in fs/unsupported.c - with the fs/unsupported.h header
file.

This also gives us a central place to document (and argue) whether an
unimplemented ioctl() should return ENOTTY or EBADF (I think the former).