Revert "Exclude src/ from include path"

This reverts commit bd3a92be4856c6c7dcdcf3a1b22d20ec95919037.
Rename limits.h to constants.h

This didn't work with MSVC as it tried to include the standard limits.h
in one if its headers.
Prevent multiple calls to public hooks

Only call the hook for the main AMX, filterscripts and anything else
should be ignored.
Add a note about HTTP()
Refine comment
Merge two conditionals