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

col.c

@@ -177,7 +177,7 @@ allocbuf(void)
 {
 	char **bp;
 
-	buff = emalloc(sizeof(*buff) * pagsize);
+	buff = ereallocarray(buff, pagsize, sizeof(*buff));
 	for (bp = buff; bp < &buff[pagsize]; ++bp)
 		*bp = emalloc(NCOLS);
 }