Building blocks for vendor-specific build of Debian packages

Building blocks for vendor-specific build of Debian packages

* scripts/Dpkg/Vendor.pm: Provide simple parser for vendor-specific
information stored in /etc/dpkg/origins/. The current vendor can
be identified through get_current_vendor() and get_vendor_info()
retrieves the information stores within those files.
* scripts/dpkg-buildpackage.pl: Setup the DEB_VENDOR environment
variable if possible.
* man/dpkg-buildpackage.1: Document the above changes.
* scripts/Dpkg/Source/Package/V3/quilt.pm (get_series_file): Use
get_current_vendor() to decide the name of the vendor-specific
series file that should be used in priority.