update Debian files, use update-alternatives

This will fix the following Lintian tags:
```
W: mintsources source: debhelper-but-no-misc-depends mintsources
W: mintsources source: debian-rules-missing-recommended-target build-indep
W: mintsources source: debian-rules-missing-recommended-target build-arch
W: mintsources source: binary-arch-rules-but-pkg-is-arch-indep
W: mintsources source: ancient-standards-version 3.7.2 (current is 3.9.5)
E: mintsources: debian-changelog-file-contains-invalid-email-address clem@vaio
E: mintsources: debian-changelog-file-contains-invalid-email-address clem@vaio
E: mintsources: debian-changelog-file-contains-invalid-email-address clem@toshiba
E: mintsources: copyright-should-refer-to-common-license-file-for-gpl
W: mintsources: copyright-without-copyright-notice
E: mintsources: dir-in-usr-local usr/local/bin/
E: mintsources: file-in-usr-local usr/local/bin/add-apt-repository
W: mintsources: file-in-unusual-dir usr/local/bin/add-apt-repository
E: mintsources: file-in-usr-local usr/local/bin/apt-add-repository
W: mintsources: file-in-unusual-dir usr/local/bin/apt-add-repository
E: mintsources: file-in-usr-local usr/local/bin/software-properties-gtk
W: mintsources: file-in-unusual-dir usr/local/bin/software-properties-gtk
E: mintsources: file-in-usr-local usr/local/bin/software-properties-kde
W: mintsources: file-in-unusual-dir usr/local/bin/software-properties-kde
```