Make use of gcc's __nonnull_if_nonzero__ attribute.
* m4/gnulib-common.m4 (gl_COMMON_BODY): Define
_GL_ATTR_nonnull_if_nonzero and _GL_ATTRIBUTE_NONNULL_IF_NONZERO.
* lib/attribute.h (ATTRIBUTE_NONNULL_IF_NONZERO): new macro.
* lib/stdlib.in.h (_GL_ATTRIBUTE_NONNULL_IF_NONZERO): New macro.
(bsearch, qsort, qsort_r): Use it.
* lib/string.in.h (_GL_ATTRIBUTE_NONNULL_IF_NONZERO): New macro.
(memcpy, memccpy, memmove, strncpy, strndup, strncat, memcmp, strncmp,
memchr, memrchr, memset, memset_explicit): Use it.
* lib/wchar.in.h (_GL_ATTRIBUTE_NONNULL_IF_NONZERO): New macro.
(wmemcpy, wmemmove, wcsncpy, wcsncat, wmemcmp, wcsncmp, wmemchr,
wmemset): Use it.
* lib/string-desc.h (sd_new_addr): Use it.