Corrected required variable double quote
Added a test for non-zero-string, corrected type ZONE instead of OPT
For being able to use e.g. "-v" to use TCP communication with the NSUPDATE_SERVER -> NSUPDATE_OPT has been added. NSUPDATE_OPT will be plainly added to the command line of nsupdate. NSUPDATE_OPT will also be saved to conf respectively read from conf