Support NUL-containing lines in sort(1)

For sort(1) we need memmem(), which I imported from OpenBSD.
Inside sort(1), the changes involved working with the explicit lengths
given by getlines() earlier and rewriting some of the functions.

Now we can handle NUL-characters in the input just fine.

util.h

@@ -76,3 +76,5 @@ long long enstrtonum(int, const char *, long long, long long);
 long long estrtonum(const char *, long long, long long);
 size_t unescape(char *);
 int mkdirp(const char *);
+#undef memmem
+void *memmem(const void *, size_t, const void *, size_t);