RFR: JDK-8301480: Replace NULL with nullptr in os/posix

[Johan Sjölen]

Hi, this PR changes all occurrences of NULL to nullptr for the subdirectory os/posix. Unfortunately the script that does the change isn't perfect, and so we
need to comb through these manually to make sure nothing has gone wrong. I also review these changes but things slip past my eyes sometimes.

Here are some typical things to look out for:

1. No changes but copyright header changed (probably because I reverted some changes but forgot the copyright).
2. Macros having their NULL changed to nullptr, these are added to the script when I find them. They should be NULL.
3. nullptr in comments and logs. We try to use lower case "null" in these cases as it reads better. An exception is made when code expressions are in a comment.

An example of this:

```c++
// This function returns null
void* ret_null();
// This function returns true if *x == nullptr
bool is_nullptr(void** x);


Note how `nullptr` participates in a code expression here, we really are talking about the specific value `nullptr`.

Thanks!

-------------

Commit messages:
 - Fix
 -  Replace NULL with nullptr in os/posix

Changes: https://git.openjdk.org/jdk/pull/12316/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12316&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301480
  Stats: 210 lines in 11 files changed: 0 ins; 0 del; 210 mod
  Patch: https://git.openjdk.org/jdk/pull/12316.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12316/head:pull/12316

PR: https://git.openjdk.org/jdk/pull/12316