env-driver: Quote value for assignment in eval.

This permits using path_add when the environment variable already contains
entries with characters that break shell quoting rules.

Technically we'd need to pipe to sed or similar to re-quote single quotes,
hopefully that won't come up...

sample-kde-env-master.sh

@@ -72,10 +72,10 @@ path_add()
 {
     eval curVal=\$'{'$1'-}'
 
-    if [ -n "$curVal" ]; then
-        eval "$1"="$2:$curVal";
+    if test -n "$curVal"; then
+        eval "$1"="$2:'$curVal'";
     else
-        eval "$1"="$2"
+        eval "$1"="'$2'"
     fi
 }