Support for adding REG_MULTI_SZ values through reg.py

[gabrielg5]

This PR implements #1720

Changing -vd argument to append

add_parser.add_argument('-vd', action='append', metavar="VALUEDATA", required=False, help='Specifies the registry '
                           'value data that is to be set. In case of adding a REG_MULTI_SZ value, set this option once for each '
                           'line you want to add.', default=[''])

Running the example with a command like this, ie:

┌──(xxx㉿XXX)-[~/impacket/examples]
└─$ python reg.py <domain>/<user>@<ip> add -v multistring -vt REG_MULTI_SZ -keyName "HKLM\SOFTWARE\test" -vd first -vd the_second -vd "with whitespaces"

[anadrianmanrique]

merged. Thanks