hush: fix some quoted variable expansion issues

The following shell command fails:

if test -z "$x"; then echo "zero"; else echo "non-zero"; fi

(assuming $x does not exist, it prints "non-zero" rather than "zero").

... since "$x" expands to nothing, and the argument is completely
dropped, causing too few to be passed to -z, causing cmd_test() to
error out early.

This is because when variable expansions are processed by make_string(),
the expanded results are concatenated back into a new string. However,
no quoting is applied when doing so, so any empty variables simply don't
generate any parameter when the combined string is parsed again.

Fix this by explicitly replacing quoting any argument that was originally
quoted when re-generating a string from the already-parsed argument list.

This also fixes loss of whitespace in commands such as:

setenv space " "
setenv var " 1${space}${space} 2 "
echo ">>${var}<<"

Reported-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>
---
v2:
* Save the "nonnull" value from the argument parser, and pass this to
  make_string(), so that it only quotes the values in the resultant
  string if the original value was quoted.
* Add some more unit tests to ensure that whitespace in quoted text is
  expanded and re-concatenated without loss.
* Revert accidental s/SUBSTED_VAR_SYMBOL/q/
unit-test: clean up evironment after Hush tests

Delete the temporary variables that are used to save unit-test results
from the environment after running the test. This prevents polluting
the environment, or growing it too much.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>