MINOR: fd: implement an optimised my_closefrom() function

MINOR: fd: implement an optimised my_closefrom() function

The idea is that poll() can set the POLLNVAL flag for each invalid
FD in a pollfd list. Thus this function makes use of poll() when
compiled in, and builds lists of up to 1024 FDs at once, checks the
output and only closes those which do not have this flag set. Tests
show that this is about twice as fast as blindly calling close() for
each closed fd.