xscope fixes

[Thomas Klausner]

When updating the pkgsrc patches to 1.4.2, I noticed quite a lot of warnings.
Here are patches that remove most.

../fd.c: In function ‘MainLoop’:
../fd.c:432:1: warning: function might be candidate for attribute ‘noreturn’ [-Wsuggest-attribute=noreturn]
  432 | MainLoop(void)
      | ^~~~~~~~

is unfixed since I could mark it _X_NORETURN but then I'd have to remove
the "return 0" at the end too -- is tha tok?