Add test coverage and generate report with lcov

Enable coverage with --enable-coverage.
"make coverage" will create the report locally,
"make coverage-sync" will upload the report to
http://www.freedesktop.org/software/systemd/coverage/.

Requires lcov version 1.10 to handle naming in systemd and to
use the --no-external option.

[zj: make the coverage at least generate something with
     separate build dir, simplify rules a bit: all errors
     are mine. ]
test: add trivial test for syscall table and extend table tests to error paths