libc: add header for ffsl and ffsll

We have an implementation of those functions that external users can call at
will.  But because they are not anywhere in our headers, OSv users will not
find the correct function signature. Man page says:

       #include <string.h>

       int ffsl(long int i);

       int ffsll(long long int i);

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
poll: fix problem with events

I have noticed a bug while testing poll today. While we are constructing the
pfd array, we correctly emplace the fd of the ith element, but events and
revents always comes from the first. That causes problems if the fds in the set
are waiting for different events.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>