Use standard C offsetof() for struct offset calculations

[seanm]

• The NT_OFF() macro was basically doing what offsetof() does, it probably predates it.
• Removed SHOW_STRUCT_OFFSET macro because nothing uses it, not even in comments.
• Changed NT_SFILL macro to use offsetof(). This required a change to its first parameter to provide the stuct name, instead of an actual variable point to a struct. Since this macro is in a header, maybe someone is using it as public API so also renamed it to NT_FILL(). Updated all call sites. Since the new formulation only needs the struct name, also removed the instantation of a struct on the stack.