Implement reallocarray()

Stateless and I stumbled upon this issue while discussing the
semantics of read, accepting a size_t but only being able to return
ssize_t, effectively lacking the ability to report successful
reads > SSIZE_MAX.
The discussion went along and we came to the topic of input-based
memory allocations. Basically, it was possible for the argument
to a memory-allocation-function to overflow, leading to a segfault
later.
The OpenBSD-guys came up with the ingenious reallocarray-function,
and I implemented it as ereallocarray, which automatically returns
on error.
Read more about it here[0].

A simple testcase is this (courtesy to stateless):
$ sbase-strings -n (2^(32|64) / 4)

This will segfault before this patch and properly return an OOM-
situation afterwards (thanks to the overflow-check in reallocarray).

[0]: http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man3/calloc.3

cut.c

@@ -56,7 +56,7 @@ parselist(char *str)
 		if (*s == ',')
 			n++;
 	}
-	r = emalloc(n * sizeof(Range));
+	r = ereallocarray(r, n, sizeof(*r));
 	for (s = str; n; n--, s++) {
 		r->min = (*s == '-') ? 1 : strtoul(s, &s, 10);
 		r->max = (*s == '-') ? strtoul(s + 1, &s, 10) : r->min;