Automated testing

[wmanley]

This branch includes adding systemd socket activation support and a simple set of automated tests which rely on such support. These tests have already been useful for finding a long standing bug in polipo (see 100ed994).

I believe automated tests are a great way to make maintainership less intimidating as with a sufficiently developed test suite less domain knowledge is required to maintain the software. You could make the argument that the domain knowledge is then encoded in the tests.

Although this branch introduces systemd socket activation support it does not introduce any dependency on systemd, it just implements the LISTEN_FDS interface. This is a completely independent implementation of the systemd interface that avoids including sd_daemon.{c,h} so no systemd code is included. Indeed, I don't even use polipo with systemd, I just needed a way of passing an already existing listening socket to polipo so decided to re-use an existing interface for that purpose.