Improved error/logged messages when namespace clone fails.

* Insert additional newline before logged messages of ERROR or higher level (vastly improves readability).
* Add a WARN about unprivileged configurations lacking subid map configuration.
* Include flags when namespace clone fails.

Example output:

      lxc-start 1405048807.231 INFO     lxc_start_ui - using rcfile /home/lxc_user/.local/share/lxc/lxc_user/config
      lxc-start 1405048807.231 WARN     lxc_log - lxc_log_init called with log already initialized
      lxc-start 1405048807.231 INFO     lxc_lsm - LSM security driver nop
      lxc-start 1405048807.231 DEBUG    lxc_start - sigchild handler set
      lxc-start 1405048807.231 DEBUG    lxc_console - opening /dev/tty for console peer
      lxc-start 1405048807.231 DEBUG    lxc_console - using '/dev/tty' as console
      lxc-start 1405048807.231 DEBUG    lxc_console - 10610 got SIGWINCH fd 9
      lxc-start 1405048807.231 DEBUG    lxc_console - set winsz dstfd:6 cols:211 rows:48
      lxc-start 1405048807.231 INFO     lxc_start - 'lxc_user' is initialized
      lxc-start 1405048807.231 WARN     lxc_start - Running with euid of 40000 but not CLONE_NEWUSER, check your config files.
      
      lxc-start 1405048807.232 ERROR    lxc_start - failed to clone with flags 0x20000011 : CLONE_NEWUSER unset
      
      lxc-start 1405048807.232 ERROR    lxc_start - Operation not permitted - failed to clone
      lxc-start 1405048807.232 DEBUG    lxc_start - Dropping cap_sys_boot
      lxc-start 1405048807.232 INFO     lxc_cgroup - cgroup driver cgroupfs initing for lxc_user
      lxc-start 1405048807.232 INFO     lxc_start - failed to pin the container's rootfs
      
      lxc-start 1405048807.232 ERROR    lxc_namespace - failed to clone (0x6c020000): Operation not permitted
      
      lxc-start 1405048807.233 ERROR    lxc_start - Operation not permitted - failed to fork into a new namespace
      
      lxc-start 1405048807.233 ERROR    lxc_start - failed to spawn 'lxc_user'
      
      lxc-start 1405048807.233 ERROR    lxc_start_ui - The container failed to start.
      
      lxc-start 1405048807.233 ERROR    lxc_start_ui - Additional information can be obtained by setting the --logfile and --log-priority options.