This patch makes some of the memory manipulation performed by psql a

This patch makes some of the memory manipulation performed by psql a
little more sane. Some parts of the code was using a static function
xmalloc() that did safe memory allocation (where "safe" means "bail
out on OOM"), but most of it was just invoking calloc() or malloc()
directly. Now almost everything invokes xmalloc() or xcalloc().