Summary:
Change NULLs to nullptrs.

Facebook:
I was tired of seeing lint errors for NULL -> nullptr, so I
fixed it.

How: modified flint's tokenizer to output lines of the form

sed -i '__LINE__s|\<NULL\>|nullptr|g' __FILE__

for each NULL token encountered.

Ran the sed lines, restricted to filepaths with extensions
h,hpp,cc,cpp,tcc. Did not apply to FacebookUpdate, due to weird
formatting; hphp, to be done in a separate diff; payment, due to a large
number of generated text which didn't play well with my flow; and
memcache, which has lots of .h's that cannot contain nullptr's because
they are included from .c's.

@bypass-lint

Test Plan:
fbconfig -r common folly && fbmake opt && fbmake runtests_opt
^ of the 4k+ test cases, about two dozen are failing. Slightly more
failed in master than with my diff applied.

arc unit

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1234261