params: handle quotes properly for values not of form foo="bar".

When starting kernel with arguments like:
  init=/bin/sh -c "echo arguments"
the trailing double quote is not removed which results in following command
being executed:
  /bin/sh -c 'echo arguments"'

Reported-by: Arthur Gautier <baloo@gandi.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>