bectl(8): Check jailparam_* return values

bectl(8): Check jailparam_* return values

Previous iteration of this assumed that these won't fail because we've
already setup the jail param to this point, but the allocations could still
fail in pretty bad conditions.

Admit that it's possible and return (ENOENT, EINVAL, ENOMEM, or 0) when
deleting arguments. EINVAL shouldn't happen since we're passing optarg;
which may satisfy *optarg == '\0' but never optarg == NULL.

CID:		1394885, 1394901