Remove #include <unistd.h> from various files that use getpid(). Instead provide platform'ized version of getpid()

[gapisback]

This cleanup comment comes out of the review cycle for PR #568 . See this comment from @rtjohnso

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I think all the other instances of #include were introduced by you in the previous PR for multi-process support. We need to clean them all out by platformizing getpid.

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Solution that will be attempted:

• Add platform_getpid() in platform_linux/platform_inline.h, defined as OS getpid()
• Use platform_getpid() everywhere getpid() is being used now.
• Will need to #include <unistd.h> in above .h

This seems like it will work ... try it out.

[gapisback]

@rtjohnso -- This is the clean-up issue you raised. Let me know if the proposed solution is acceptable to you. I'll get to this after part-3 lands.