Apply patches before building source packages when needed

Apply patches before building source packages when needed

* scripts/Dpkg/Source/Package/V2_0.pm (do_extract): Create
debian/patches/.dpkg-source-applied containing the list of patches
applied during extraction.
(prepare_build): apply patches before trying to build a source package
when it's clear that they have not been applied because
debian/patches/.dpkg-source-applied doesn't exist.
* scripts/Dpkg/Source/Package/V3_0/quilt.pm: Likewise. If a .pc directory
is present, then use "quilt unapplied" to check if all patches have been
applied.
* man/dpkg-source.1: Document the above changes.