Change slurm_xrealloc() to slurm_recalloc(). Add xrecalloc() macro.

Change it so that we can resize arrays conveniently. Renaming
similarly to slurm_xcalloc().

This name is something of my own invention - there is no direct libc
equivalent that will resize allocations and ensure the contents are
zero'd out, let alone for arrays, and reallocarray() itself is
already non-standard.

Add an extra bool argument so we can introduce try_xcalloc() while here.