Don't include <osv/poll.h> in <osv/file.h>

We can't include osv/poll.h from osv/file.h. It's not needed there,
and causes a mess now that I added a use of BSD's "struct mtx" to
poll.h (because it turns out we have code using <osv/file.h> and using
the name "mtx" for something else).

After removing the unneeded include of <osv/poll.h> from osv/file.h,
we need to add include <sys/poll.h> to a few additional source files
(note include of sys/poll.h, not osv/poll.h).