* scripts/controllib.pl, scripts/dpkg-checkbuilddeps.pl: Moved dependency

* scripts/controllib.pl, scripts/dpkg-checkbuilddeps.pl: Moved dependency
  parsing logic from dpkg-checkbuilddeps.  The dpkg-checkbuilddeps parser
  didn't support empty fields, while the controllib parser did.
* scripts/controllib.pl:
  * Dependency fields are now parsed into a nested list structure.
  * All dependency fields now support [arch] constructs.  Those that don't
    apply for the current host_arch are removed during parsing.
  * Comment lines(those that start with '#') are ignored during parsing.
  * Store the original field's casing with 'o:' prepended in %fi.
* scripts/dpkg-checkbuilddeps.pl:  Because of the above controllib.pl
  changes, this script is much simpler now.
* scripts/dpkg-gencontrol.pl:
  * Use the original casing of field names when reporting unknown fields.
* scripts/dpkg-gencontrol.pl, scripts/controllib.pl: After opening files,