Fix handling of extcap boolflags which defaults to true.

Example:
arg {number=0}{call=--test}{type=boolflag}{default=true}

Before this change --test was never added to argument list (no matter if
user left it selected or explicitly deselected it).

After this change --test will be added to argument list unless user
explicitly deselects it.

Change-Id: Ia5bc11f900b03e630aba882ef918dcb7f0b79291
Reviewed-on: https://code.wireshark.org/review/4618
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Pascal Quantin <pascal.quantin@gmail.com>