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

util.h

@@ -26,6 +26,8 @@ void apathmax(char **, size_t *);
 void *ecalloc(size_t, size_t);
 void *emalloc(size_t);
 void *erealloc(void *, size_t);
+void *reallocarray(void *, size_t, size_t);
+void *ereallocarray(void *, size_t, size_t);
 char *estrdup(const char *);
 char *estrndup(const char *, size_t);
 void *encalloc(int, size_t, size_t);