hush: Don't parse the contents of a dereferenced var

When a variable which contains a user-supplied value is dereferenced
(e.g. to be echo'ed), make sure that the value is not further parsed
by hush.

Set the hush local variable "HUSH_NO_EVAL=1" to enable this behavior.

Without this patch, a sequence like this occurs:

	Panda # env set my_user_string Bob\'s favorite device
	Panda # print my_user_string
	my_user_string=Bob's favorite device
	Panda # echo $my_user_string
	syntax error hush.c:3007

With this patch, it looks like this:

	Panda # HUSH_NO_EVAL=1
	Panda # env set my_user_string Bob\'s favorite device
	Panda # print my_user_string
	my_user_string=Bob's favorite device
	Panda # echo $my_user_string
	Bob's favorite device

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>