[v3 patch] Fix some Filesystem TS operations

[Jonathan Wakely]

Testing revealed a few bugs in how I handled paths that don't exist.
The new __gnu_test::nonexistent_path() function is a bit hacky but
should be good enough for the testsuite.

I've also made filesystem::temp_directory_path() check the TMP,
TEMPDIR and TEMP environment variables, as suggested in
[fs.op.temp_dir_path] at
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4099.html#fs.op.temp_dir_path
rather than my original implementation which only checked TMPDIR
(because I think that's the only one mentioned by POSIX). I'm not
unsure whether it's better to follow the non-normative note or only
check for the POSIX name. Anyone else have any thoughts?

Tested x86_64-linux, committed to trunk.
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