dpkg-buildpackage: use space as separator in DEB_BUILD_OPTIONS

dpkg-buildpackage: use space as separator in DEB_BUILD_OPTIONS

* scripts/Dpkg/BuildOptions.pm (parse, set): Use space as the
official separator in DEB_BUILD_OPTIONS. Check for validity of
flags and print a warning if a bad option is detected. Rewrote
the logic of set() to avoid adding options twice in non-overwrite
mode.