Refactor lxc-snapshot

Refactor lxc-snapshot

- lxc_snapshot.c lacked necessary members in the associated lxc_arguments struct
  in arguments.h. This commit extends the lxc_arguments struct to include
  several parameters used by lxc-snapshot which allows a rewrite that is more
  consistent with the rest of the lxc-* executables.
- All tests have been moved beyond the call to lxc_log_init() to allow for the
  messages to be printed or saved.
- Some small changes to the my_args struct. (The enum task is set to
  SNAP (for snapshot) per default and variables illustrating the usage of the
  command line flags are written in all caps.)
- arguments.h has been extended to accommodate a rewrite of lxc-clone

Signed-off-by: Christian Brauner <christianvanbrauner@gmail.com>