A typical use case is like this:
printf("%s: %s\n", tuklib_mask_nonprint(filename), strerror(errno));
tuklib_mask_nonprint() may call mbrtowc() and malloc() which may modify
errno. If errno isn't preserved, the error message might be wrong if
a compiler decides to call tuklib_mask_nonprint() before strerror().
Fixes: 40e573305535960574404d2eae848b248c95ea7e
c405264c03