Import reallocarray() from libX11 (originally from OpenBSD)

Wrapper for realloc() that checks for overflow when multiplying
arguments together, so we don't have to add overflow checks to
every single call.  For documentation on usage, see:
http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man3/calloc.3



Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>