dpkg-gencontrol: accept "-c-" to read the control file from stdin

dpkg-gencontrol: accept "-c-" to read the control file from stdin

* scripts/Dpkg/Control.pm (parse, parse_fh, new): Add a new function
parse_fh() to be able to parse the control file from an arbitrary
file handle. Change parse() to use it and modify new() to parse
STDIN instead of a real file if the parameter is "-".