Maintenance: Removed most NULLs using modernize-use-nullptr (#1075)

Maintenance: Removed most NULLs using modernize-use-nullptr (#1075)

Applying clang-tidy modernize-use-nullptr also converted many
"anonymous" 0s into nullptr.

Clang-tidy uses clang AST, so we can only modernize code what clang can
compile. Fortunately, it is fairly easy to compile most of Squid code,
eliminating ~90% of NULLs in actively maintained code (excluding
comments and Windows-specific code) and ~73% of all NULLs.

TODO: Consider eliminating all remaining C++ NULLs using a simple script
that does not understand C++ syntax, assuming that the remaining cases
are simple enough to avoid problems that a compiler cannot detect.