cmd/power: Set correct container name and ensure full command executed

[vancluever]

This fixes a few issues that are preventing shutdown and friends from
behaving correctly:

• The command name, which is being used to determine via what command it
  was being called (ie: shutdown, reboot, or halt) was not being parsed
  for absolute paths. This was preventing certain logic from being handled
  (example: enforcing a static time value of now for shutdown), but more
  problematically was the fact that it was being used as the container
  name being passed to runDocker, the function that launches the
  independent shutdown container. This was causing the shutdown container
  to fail as something like /sbin/shutdown is not a valid name for a
  container. The logic to parse out the base command being run is actually
  present in runDocker, but does not run if a name is supplied to the
  function.

• Further, the command line was not being parsed in the shutdown
  container if the name supplied to runDocker was non-empty. Rather, the
  full command to run just became the name of the container. Hence,
  something like /sbin/shutdown -h now became just shutdown, executing
  the default power off behaviour for all actions (including reboots).

• Further to this, open-vm-tools expects /sbin/shutdown -h now to be a
  valid command to halt the system, which was not being recognized as the
  only recognized short-form halt flag in shutdown was its capital version
  (-H).

This fixes these three issues by parsing out the base of the called
command before sending it to reboot, using all of os.Argv as the command
line to run regardless of if a name was set for the container or not,
and finally adding the lowercase -h switch to the "shutdown" form of
this command ("halt" is still uppercase only).

Fixes #2121.
Fixes #2074.