Add XtReallocArray() for overflow checking of multiplied args

Uses reallocarray() if available, otherwise checks for overflow itself,
if overflow is possible (i.e. in ILP32 & ILP64 environments, but not LP64
with 32-bit ints).

Includes unit tests and XtMallocArray() helper macro.

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