Replace ERROR() macro with inline function. In-tree gcc cannot tolerate

Replace ERROR() macro with inline function. In-tree gcc cannot tolerate
the construct like printf("%\s", NULL) resulting from macroexpand of
ERROR(u, NULL), making it impossible to use LIBUFS_DEBUGGING.

With inline function, compiler cannot detect the NULL argument to
known function and does not try to convert it into puts().

In collaboration with:	pho