use poll() to find open fds, removes malloc() after fork()

Resolves issue #173: libdbus calls malloc() after fork(), causing clients to
deadlock

calling malloc() after fork is undefined behaviour if the process
multi-threaded. The new code is a commonly used performance workaround for
the fork-and-close pattern and uses only async-signal safe functions. See
signal-safety(7).
11 jobs for poll in 10 minutes and 32 seconds (queued for 3 seconds)
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