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It might be that stdin was set to O_NONBLOCK by someone else, and we
should handle this case.  (this happens with GHCi, I'm not quite sure why)

Now, we only set O_NONBLOCK on file descriptors that we create
ourselves.  File descriptors that we inherit (stdin, stdout, stderr)
are kept in blocking mode.  The way we deal with this differs between
the threaded and non-threaded runtimes:

 - with -threaded, we just make a safe foreign call to read(), which
   may block, but this is ok.

 - without -threaded, we test the descriptor with select() before
   attempting any I/O.  This isn't completely safe - someone else
   might read the data between the select() and the read() - but it's
   a reasonable compromise and doesn't seem to measurably affect
   performance.

The nhc98 does not have true exceptions, but these additions should be
enough infrastructure to pretend that it does.  Only IO exceptions will
actually work.

A first attempt at removing gratuitous cycles in the base package.
I've removed the useless module GHC.Dynamic, which gets rid of a cycle;
and trimmed off various unnecesary imports.

This also fixes the IsString import problem.

fixes cg026

Avoids the existing autoconf stuff, by introducing an auxiliary module
called NHC.PosixTypes that uses hsc2hs, which is then simply re-exported
from System.Posix.Types.

This may be responsible for the x86_64/Linux nightly build failing.