Let dpkg-source ensure (as good as possible) that all

Let dpkg-source ensure (as good as possible) that all
build source packages can also be unpacked.
Closes: #6820, #7014
* scripts/controllib.pl:
  (checkversion) add generic check for valid version numbers
  (checkpackagename) add generic check for valid package
  names
  (readmd5sum) add generic function to extract md5sum from
  md5sum program output
  (setsourcepackage) call checkpackagename on new value
* scripts/dpkg-source.pl: Use the new checks added to
  controllib to ensure validity of version and packagename
  on build, too. Previously this was only done on
  unpack.
  Test on build if directories added by diff already exist with
  other type in the original source since we already tested that
  on unpack.
  (addfile): Test if files are added
  twice. Should not happen but as we error out on unpack
  better make sure it doesn't.