libkvm: Plug couple of memory leaks and check possible calloc(3) failure

First, r204494 introduced dpcpu_off in struct __kvm and it was allocated
from _kvm_dpcpu_init() but it was not free(3)'ed from kvm_close(3).
Second, r291406 introduced kvm_nlist2(3) and converted kvm_nlist(3) to
use the new function but it did not free the temporary buffer.
Also, check possible calloc(3) failure while I am in the neighborhood.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29019