Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

GNU glibc 2.23 and later deprecate `readdir_r` in favour of plain old
`readdir` which in some upcoming POSIX standard is going to required to be
re-entrant.

Eventually we want to drop `readder_r` all together, but want to be
compatible with older unixen which may not have a re-entrant `readdir`.

Solution is to make systems with *known* re-entrant `readir` use that and
use `readdir_r` whereever we have it and don't *know* that `readdir` is
re-entrant.

Closes: https://github.com/haskell/unix/issues/70

This has the side-effect of making two more modules `Safe`-inferred

This improves on 2ddf4b2b

This is now possible since we now use `AC_USE_SYSTEM_EXTENSIONS`, which
indirectly enables _XOPEN_SOURCE

We haven't properly supported Windows for some time now, and we wouldn't
have any way to test anyway, since GHC doesn't support Cygwin anymore
either.

Follow-up to 11eb5aab

This fixes #50

Erik de Castro Lopo authored Dec 19, 2014 and Herbert Valerio Riedel committed Dec 19, 2014

It seems these two signals have not been working since at least
2009. Detection of these signals seems to have never been added to
the configure.ac script and the code guarded by #ifdef then bit-rotted
(the idiom used to handle these signals seems to have been abandoned
for something simpler/better in 2009). This fix simply handles these
signals the same way the other signals are handled in
System/Posix/Signals.hsc.

Closes #30 and #31

The Bool field of Terminated is new, as is the documentation:

data ProcessStatus
   = Exited ExitCode        -- ^ the process exited by calling
                            -- @exit()@ or returning from @main@
   | Terminated Signal Bool -- ^ the process was terminated by a
                            -- signal, the @Bool@ is @True@ if a core
                            -- dump was produced
   | Stopped Signal         -- ^ the process was stopped by a signal
   deriving (Eq, Ord, Show)

This is an API change, hence will need a major version bump.

Fixes validate on amd64/Linux with:

SRC_CC_OPTS += -Wmissing-parameter-type
SRC_CC_OPTS += -Wold-style-declaration
SRC_CC_OPTS += -Wold-style-definition

functions need to be given a new name, and the header files contain
some __asm hackery in order to let the program call the correct function.

This mean that you need to use the header files in order to call the
correct system functions, which prevents things like "foreign import ccall" from working.

Ghc solves this with wrapper functions for some of the renamed functions,
but it has not been updated for newer versions of NetBSD that has recently
versioned some more functions.

The attached patches introduces wrapper functions for all currently
NetBSD-versioned functions used in libraries/unix.  Solves ~20 testsuite
failures.

Contributed by: Krister Walfridsson <krister.walfridsson@gmail.com>

If they are included into a C file which also has certain symbols
defined, then the behaviour of the HsUnix.h functions can change
(e.g. lstat can become the 32bit, rather than 64bit, version).

More POSIX bits... we're getting there.