Backports for 2.7.3 (#235)

* Add regression test for #218

https://github.com/haskell/unix/issues/218



Error was:

uncaught exception: IOException of type InvalidArgument
semTrywait: invalid argument (Bad file descriptor)

* Fix 'semTrywait: invalid argument (Bad file descriptor)' wrt #218

* Add regression test for putEnv wrt #68

* Synchronize ByteString and String modules

And fix free-bug in 'putEnv'. Fixes #68

* Add cabal.project

* Fix the error handling of posix_fallocate other than FreeBSD

This commit introduces a CPP guard to take care of the difference
between OSes of the errors of posix_fallocate.

On Linux and NetBSD, posix_fallocate reports error by returning an error
number. errno is not set.
On the other hand, on FreeBSD, posix_fallocate returns -1 and sets
errno.

The existing code could handle FreeBSD-style errors only.

* Remove obsolete FreeBSD-style error handling

The FreeBSD-style error (return -1, set errno) is obsolete as of
FreeBSD 11.0; it now reports errors as same as the other OSes.

* Future-proof glibc version check

* Resource: Fix warning in case of no RLIM_SAVED_{CUR,MAX}

This previously failed to build with a warning on FreeBSD 11.2

* Bump to 2.7.3

* Add Eq and Show instance to BaudRate

Needed for test suite

* Add CI

* Tighten lower base bound to only allow GHC-8.2+

Co-authored-by: matil019 <yohashi1991@gmail.com>
Co-authored-by: Viktor Dukhovni <ietf-dane@dukhovni.org>
Co-authored-by: Ben Gamari <ben@smart-cactus.org>