maint: add function attributes to .h files

maint: add function attributes to .h files

Add _GL_ATTRIBUTE_NONNULL, _GL_ATTRIBUTE_MALLOC,
_GL_ATTRIBUTE_DEALLOC, _GL_ATTRIBUTE_DALLOC_FREE,
_GL_ATTRIBUTE_RETURNS_NONNULL to .h files when appropriate.
* gl/lib/mbsalign.h, gl/lib/randperm.h, src/chown-core.h:
Include stdlib.h, for the benefit of _GL_ATTRIBUTE_DALLOC_FREE.
* gl/lib/randread.c (randread_free_body): New static function.
(randread_new, randread_free): Use it.
* src/copy.c (valid_options): Remove assert that is no longer
needed because it is now checked statically.