[Cleanup] Use C++11 nullptr instead of NULL

[Fuzzbawls]

Scripted diff to replace all NULL macros with the C++11 nullptr keyword

-BEGIN VERIFY SCRIPT-
sed -i 's/\<NULL\>/nullptr/g' src/*.cpp src/*.h src/*/*.cpp src/*/*.h src/qt/*/*.cpp src/qt/*/*.h src/qt/pivx/*/*.cpp src/qt/pivx/*/*.h src/wallet/*/*.cpp src/test/*/*.cpp src/wallet/*/*.h
sed -i 's/Prefer nullptr, otherwise SAFECOOKIE./Prefer NULL, otherwise SAFECOOKIE./g' src/torcontrol.cpp
sed -i 's/tor: Using nullptr authentication/tor: Using NULL authentication/g' src/torcontrol.cpp
sed -i 's/METHODS=nullptr/METHODS=NULL/g' src/test/torcontrol_tests.cpp src/torcontrol.cpp
sed -i 's/nullptr certificates/NULL certificates/g' src/qt/paymentserver.cpp
sed -i 's/"nullptr"/"NULL"/g' src/torcontrol.cpp src/test/torcontrol_tests.cpp
-END VERIFY SCRIPT-

[random-zebra]

utACK a3346e2

[PeterL73]

tACK 54e1168

Nit: src/qt/paymentserver.cpp does not contain NULL certificates so sed -i 's/nullptr certificates/NULL certificates/g' src/qt/paymentserver.cpp could be skipped

[panleone]

utACK a3346e2

[Fuzzbawls]

rebased on current master branch to resolve conflicts. lets prioritize this one so I don't have to keep rebasing it further. Scripted-diff single commit should be an easy review.

[panleone]

re-utACK 58e254e
changes are consistent with the script

[Liquid369]

tACK 58e254e