Some of our files include headers inside an "extern C"  declaration.  The
problem with that eventually we will have definitions with C linkage - since
they are inside extern "C" - being included in places where __cplusplus is
defined. I specifically worked around this int the sx_xlock implementation,
but it is starting to create bigger problems. Fix it by making all headers
usable from whichever place by themselves, instead of relying in extern inclusions
in the cpp files

Mostly to get the X bits on directories right for lookups, as well
as checks for hard readonly files.

It may not be available yet.  Change the mode checks to use zp->z_mode,
and remove the devvp checks as we do not support device nodes outside
of devfs.

We might not have a vnode assigned to a znode yet when doing access
checks from lookup.

Because this is arch specific, I am adding it to a newly created file in
arch/x64. I am making it available to BSD through netport for the lack of a
better place

ZFS expects vdev providers to work asynchronously and waits in higher level
code when needed.

Dear BSD developers: a long is a long, not an int.
Because of that define, the struct linux uses for its ifreq ioctl will
be of a different size (it is fine in 32-bit machines of course), causing
our network requests to get borked.

This operation is very different from FreeBSD and Solaris because our
VFS uses create just for the actual entry creation and not for opening
the file, similar to how Linux splits the operation.  A lot of code
that is already handled in vnop_open or the VFS thus can go away.

This is required for write operations.  For now we don't actually replay
it yet, as that requires a lot more hairy OS-specific code.

This one will only show up in non-debug builds for some reason.

Now that we have a reliable number of CPUs indicator the implementation
is trivial.

This patch fixes two bugs in so_wake_poll(), which caused us missing
some poll wakeups, resulting in poll()s that never wake up. This can be
seen as a hang in the following simple loop exercising memcached:
         (while :; do; echo "stats" | nc 192.168.122.100 11211; done)

The two fixes are:

1. If so_wake_poll() decides *not* to call poll_wake() - because it sees
   zero data on this packet - it mustn't reset the SB_SEL flag on the
   socket, or we will ignore the next event even when it does have data.

2. To see if the socket is readable, we need to call soreadable(), not
   soreadabledata() - the former adds the connection close event to the
   readability. See sopoll_generic(), which also sets a readability
   event in that case.

verbatim header files import, plus the code in build.mk to find them

wakeup() and wakeup_one() dropped the lock before waking the thread(s).
But if a thread timed out at the same time we are planning to wake it,
and if the thread imediately exited, our attempt to wake() it could crash.

Therefore wakeup() needs to continue holding the lock until after the
wake(). When msleep() times out, before it returns it reacquires the
lock, ensuring it cannot return before a concurrent wakeup() does
its wakes.

Note that in the ordinary wakeup() case (not racing with a timeout),
we are *not* negatively affected by holding the lock while wake()ing
because the waking msleep() does not try to reacquire the lock in the
non-timeout case.

Additionally, we use wake_with() instead of separate set and wake
instructions. Without wake_with() we have the (exceedingly rare)
possibility that merely setting _awake=true causes msleep() to
return, and its caller then decides to exit the thread, the
wake() right after the _awake=true would crash.