Add argument documentation for Env modules

Fixes #76

Fix segfault from inconsistent macro use.

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

Don't use readdir_r if deprecated

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

Fix travis build

`no_stdin` is no longer necessary, and has been removed from the
testsuite driver

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

This improves on 2ddf4b2b

As we don't support Windows, there's no benefit in going via
`__hscore_open()`

There's been some subtle API changes that may be useful to detect via
`MIN_VERSION_unix(2,7,2)`, consequently perform a minor version bump
relative to the last release.

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

It's very unlikely the current `unix` code stands any chance of even
remotely work with Hugs...

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.

This avoids incompatible-pointer warnings from the c-compiler when using
`CApiFFI`