Remove NeXTSTEP references

Remove trailing whitespace as well.
Remove Atari MiNT references
Remove OS/2 references

Paul Slootman was removed from the top of os_unix.c as OS/2 is no longer
supported, but is still credited in runtime/doc/intro.txt.
Merge pull request #1491 from Pyrohh/os2-and-misc-cleanup

Remove MiNT, NeXTSTEP, OS/2 References