libnv: switch fd_wait() from select(2) to poll(2)

[oshogbo]

Summary

The previous implementation used FD_SET() on a stack-allocated fd_set,
which is an out-of-bounds write whenever the socket fd is >= FD_SETSIZE
(1024).

poll(2) takes an array indexed by slot rather than by fd value, so it
has no FD_SETSIZE limit.

Reported by: Joshua Rogers