Dpkg::Source: don't handle the compression level explicitely

Dpkg::Source: don't handle the compression level explicitely

* scripts/Dpkg/Source/Archiver.pm: Drop set_compression_level() and always
use the default compression level (from Dpkg::Source::Compressor).
* scripts/Dpkg/Source/Compressor.pm: New set_default_compression() and
set_default_compression_level() to change the default values of those
parameters.
* scripts/dpkg-source.pl: Change the default compression and the default
compression level globally. Replace forkgzipread(), forkgzipwrite() and
reapgzip() by direct usage of Dpkg::Source::Compressor in a way that it
inherits the compression level automatically. Also simplify some
subprocess execution by using Dpkg::IPC::fork_and_exec().