debian: Do not backup nonexistent database files

The arch database only exists if the user has more than one architecture
configured.

Closes: #745592, #745651
debian: Use a variable to track the alternatives database name
debian: Do not emit tar warnings from dpkg cron file

Ref: #745592, #745651
build: Use 'rm -f' instead of implicit GNU make RM variable
build: Generate a C code coverage text report on stdout

Print a text report to stdout, like with the Perl coverage.
build: Add descriptive titles before each coverage section
build: Use --coverage instead of -fprofile-arcs and -ftest-coverage

Noticed while investigating the lack of branch coverage. It appears the
option is supported for a very long time, at least since gcc 4.1.x.
build: Use LCOV_OPTS in all lcov related invocations

Switch from --checksum to --rc geninfo_checksum=1 so that it can be
passed to all commands, even the ones that do not recognize --checksum.
build: Enable back lcov branch coverage

This got disabled by default starting with lcov 1.10, enable it back
to get the full coverage report.
build: Inject a Perl coverage index entry into the lcov report

Add correct summary values, create a percentage bar, and remove the
lcov-epilog template, which was being inserted in every and each
generated lcov html file, not just the indices.

The injection should be considered fragile, as it depends on the input
report not changing its structure. But this is no worse than using the
local prolog and epilog html templates.
build: Remove top_builddir value from Perl coverage report

Do not expose the builddir in the html output.
build: Stop using a local lcov-prolog template

Newer lcov versions already produce UTF-8 encoded output, so there's no
need to use our local html prolog.

Ref: #590562
build: Add project version to the C code coverage report
Dpkg::Dist:Files: Add test cases for get_file()
Dpkg::Deps: Add test cases for deps_concat()
Dpkg::Vendor: Add test cases for get_vendor_dir()
libdpkg: Add test cases for dpkg_arch_describe()
libdpkg: Add test cases for command_get_pager()
libdpkg: Add test cases for growing command argv
libdpkg: Add test cases for unmatched str_match_end()