Communicate PIPE_BUF default value to test suite

On platforms that don't define PIPE_BUF (such as GNU Hurd), we assume a
value of 512 in src/server/fifo.c (as POSIX guarantees PIPE_BUF >= 512).
We now communicate this default value to Autotest in order to fix the
corresponding test case on such platforms.
Fix test cases for GNU Hurd

GNU Hurd doesn't like the the trick we're using in tests/test_nsca.c to
avoid having to wait until the server notices that we opened the command
file for reading.  This means that each check might block for a few
seconds on Hurd, but that's still preferable to failing test cases.
m4/aio.m4: Check whether AIO signals are delivered

At least on Debian GNU/kFreeBSD 7.0 RC1, the POSIX AIO implementation
fails to deliver completion notification signals (but seems to work
otherwise).  The ./configure script now detects this case and then
disables POSIX AIO usage.