libavrdude.h fixes and installation of libavrdude-avrintel.h

[ndim]

This cleans up libavrdude.h a bit and installs libavrdude-avrintel.h:

• libavrdude.h will create an error if LIBAVRDUDE_INCLUDE_INTERNAL_HEADERS is defined
• add comment on top of libavrdude-avrintel.h about not directly #includeing it
• install libavrdude-avrintel.h alongside libavrdude.h (both cmake and autotools)
• use proper C99 and later definitions for a C function without parameters: rettype funcname(void);
• have the libavrdude.h/libavrdude-avrintel.h combination #include all required libc headers declaring the types used in them (add stddef.h for size_t)

Test programs for testing language compatibility will come later when I figure out how to build them in cmake. A program like the following should build with all compilers and all supported language standards:

#include <libavrdude.h>

int main(void) {
  return 0;
}

[mcuee]

From github action, it seems to be good under Linux (CMake and auto-tools).

I have also tested this under macOS.

I think this PR is good to go and it finally fixes #1683.

• avrintel.h missing from installation #1683

[stefanrueger]

I have reviewed the changes in this PR, and they look good to me. Happy to merge at next mergefest (we tend to let PR lie around for a few days and normally wait to merge them until a few ones are ready)