prepare transition to poll()

usage of select() is inefficient (because a huge fd_set array has to
be initialized on each call) and insecure (because an fd >= FD_SETSIZE
will cause out-of-bounds accesses using the FD_*SET macros, and a system
can be set up to allow more than that number of fds using ulimit).
for the moment we prepared a poll-like wrapper that still runs select()
to test for regressions, and so we have fallback code for systems without
poll().

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