Fix variable names and use named constants

source/App/BitstreamExtractorApp/BitstreamExtractorApp.cpp

source/App/StreamMergeApp/StreamMergeApp.cpp

source/App/SubpicMergeApp/SubpicMergeApp.cpp

source/Lib/CommonLib/BitStream.cpp

source/Lib/CommonLib/BitStream.h

source/Lib/EncoderLib/NALwrite.cpp