dpkg-genchanges: use Package-Type control field to detect udebs

dpkg-genchanges: use Package-Type control field to detect udebs

* scripts/Dpkg/Fields.pm (find_custom_field, get_custom_field):
New function to handle custom fields (X[SBC]*-*).
* scripts/dpkg-genchanges.pl: Use Package-Type control field to
decide if a package is an udeb instead of relying on the file
extension of a generated package since that doesn't work when
generating source-only uploads for example.