Add xcalloc() macro and convert slurm_xmalloc into slurm_xcalloc() function.

Use this instead of handling the multiplication ourselves. This lets us
check for potential problems from integer overflow, and IMNSHO makes the
code look a lot tidier.

Rework xmalloc() and xmalloc_nz() macros in terms of slurm_xcalloc().

Bug 6310.