Posix adapter shouldn't rely on O_TMPFILE support

[ChristopherHogan]

Kernel versions < 3.11 and certain filesystems (e.g., lustre) don't support the O_TMPFILE flag to open. The flag is used in two ways in the POSIX adapter.

1. Hermes must treat an open with O_TMPFILE like an open with O_CREAT, since both should result in the creation of a Bucket.

if (flags & O_CREAT || flags & O_TMPFILE) {
  // ...
}

2. The POSIX adapter tests create temporary files with O_TMPFILE to test Hermes' handling of the flag.

To fix 1), I define O_TMPFILE to 0 if it's not defined. To fix 2), I only run the O_TMPFILE tests after verifying that the kernel and filesystem both support O_TMPFILE.

[hyoklee]

This is great patch for macOS users. See

https://github.com/HDFGroup/hermes/wiki/FAQ#can-i-run-hermes-on-mac

[coveralls]

Coverage decreased (-0.02%) to 81.583% when pulling a284e5a on chogan/tmpfile into 4fac80a on master.