docs: make linuxcnc.1 in the docs: target

Instead of in the default: target.
deb: provide build-arch and build-indep targets in debian/rules

These build targets are Recommended by Section 4.9 of the Debian Policy
Manual, version 3.9 (Debian Wheezy and Ubuntu Precise and newer).

The fix in this commit is the one recommended by lintian for lazy
projects like ours that don't want to bother splitting the building of
architecture-dependent and architecture-independent packages.