cleanups

1. reac_criu_file: scanf("%2s", &s) will read 3 bytes, not two.  So only
read 127 bytes, and comment that the passed-in buffer must be 128 bytes.

2. for all sprintfs which take a %s, use snprintf and check return value.

3. Check that caller is root at top of lxcapi_{checkpoint,restore} to avoid
obscure confusing errors

4. Add comment asking about error path if veth.pair is not found in
container at checkpoint.  Please answer.

5. Add a comment asking about reading pid_t from file.  Please answer.

6. check strdup() result for veth_attr.pair

Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>

Conflicts:
	src/lxc/lxccontainer.c