replace sOffset (use offsetof)

[jg1uaa]

Building DOSBox-X on OpenBSD/amd64 (clang-13), I see many -Wnull-pointer-subtracion warnings like this:

../../include/dos_inc.h:542:35: warning: performing pointer subtraction with a null pointer may have undefined behavior [-Wnull-pointer-subtraction]
        void SetBootDrive(uint8_t drv) { sSave(sDIB,bootDrive,drv); }
                                         ^~~~~~~~~~~~~~~~~~~~~~~~~
../../include/dos_inc.h:395:61: note: expanded from macro 'sSave'
#define sSave(s,m,val) SaveIt(sizeof(((s *)&pt)->m),(PhysPt)sOffset(s,m),val)
                                                            ^~~~~~~~~~~~
../../include/dos_inc.h:393:46: note: expanded from macro 'sOffset'
#define sOffset(s,m) ((char*)&(((s*)NULL)->m)-(char*)NULL)
                                             ^~~~~~~~~~~~

I think sOffset() in dos_inc.h can be replaced with offsetof() and it can reduce this warning. Is there any historical reason to keep this code?

(in OpenBSD's stddef.h, offsetof() is defined like this)

#if __GNUC_PREREQ__(4, 0)
#define offsetof(type, member)  __builtin_offsetof(type, member)
#else
#define offsetof(type, member)  ((size_t)(&((type *)0)->member))
#endif

[maron2000]

I didn't try but one of the concern is that whether the code is C++11 compatible.
The following is sited from the contribution guide.

DOSBox-X is still intended to support compilation as C++11.
 If you add new code please make sure it works as C++11. 
One way to do so is to run ./configure with --enable-force-cxx11 option added, 
and build the code.

[jg1uaa]

I checked this change does not make error to compile with gcc-8.4.0 and clang-13 on OpenBSD,
with or without configure --enable-force-cxx11.

at least, dos_inc.h has already included <stddef.h> to use offsetof().
https://github.com/joncampbell123/dosbox-x/blob/master/include/dos_inc.h#L28

[joncampbell123]

offsetof() is way older than C++11, it shouldn't be a problem.

https://en.cppreference.com/w/cpp/types/offsetof

It was there because the original DOSBox project had it too.

Perhaps DOSBox Staging might appreciate this change as well, since they also intend to modernize their code base (and sometimes even sneer at me a little for not jumping onto the C++17 bandwagon).