Update debian packaging

[SebKuzminsky]

This fixes packaging on Debian Unstable:

• Split the package build & install for architecture-specific vs architecture-independent packages, as required by Debian's build infrastructure. This includes some support in src/Makefile, but the common build targets used by humans (i.e. make and make docs) are unchanged.
• Move the G-code quick ref html files (and their .desktop files) from the linuxcnc-uspace package to the linuxcnc-doc-* packages. This is required now that the quick ref translations are generated as part of the architecture-independent build (docs), not the architecture-specific (binary) build.
• Likewise, move the LinuxCNC Developer's Guide from linuxcnc-uspace-dev to linuxcnc-doc-en, for the same reason.
• Switch debian/rules to rely more on the debhelper tools, and do less "by hand".
• Fix a bug in debian/rules that made it accidentally rebuild stuff it had just built...
• Minor simplifications to debian/configure (just because I happened to be in the neighborhood)

This PR (or something like it) is needed for the next dsc upload to debian.org. I'd appreciate feedback from folks who know debian packaging (@smoe, @petterreinholdtsen, @jepler, and anyone else with input).