d/rules: Use standard dh sequence (Closes: #1122012, 1122013)

This package currently uses debhelper with an ad-hoc sequence of steps in d/rules. This long list of steps can be shortened and greatly simplified by using the standard dh sequence.

According to diffoscope, the generated binary packages are identical to the current ones. (Except for one checksum in the .gnu_debuglink ELF section of libltdl.so.7.3.3.) Additional simplifications are possible, but they would break bit-for-bit replicability with the current binaries.

Please note that the block dealing with making the package clean for a second in-place build is no longer there. It was broken and hard to integrate as-is in the new code. I will submit a new bug describing the issue, together with an associated patch that restores and fixes that functionality on top of this MR. (This is why the test-build-validate-cleanup of the associated pipeline fails.) Support for cleaning the build environment has been rewritten from scratch using dh_assistant restore-file-on-clean.