Make *.dsc build command configurable via 'dsc:build_cmd' substitution

Instead of hardcoding the build command used for *.dsc files lets
substitute from the configuration. Adding a build.script for every package
doesn't scale.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
Don't use fakeroot-tcp for dsc builds

Make dpkg-buildpackage use its default gainroot command (fakeroot). The
performance overhead introduced by using fakeroot-tcp (see 30af55b1) is
very large. For a chmod/stat heavy use-case like a kernel build this easily
adds up to ~4000 seconds.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
Merge pull request #139 from jblunck/debian_fakeroot

Don't use fakeroot-tcp by default for *.dsc builds