Enable munin-node-c to build and run on macOS/Darwin

[smokris]

A couple small changes that enable munin-node-c to build and run on macOS/Darwin. Tested on OS X 10.10, macOS 10.13, and macOS 10.14.

(munin-plugins-c doesn't work yet, since that relies on /proc which doesn't exist on macOS. But this PR enables munin-node-c to work with other plugins.)

[steveschnepp]

Nice!

Would you be awesome by adding corresponding lines to the .travis.yml file?

Note that I'll squash the PR into only 2 commits prior to merge.

[smokris]

Sure, I modified the Travis CI configuration to also build on macOS.

[steveschnepp]

I rewrote most of your commits messages in https://github.com/steveschnepp/munin-c/commits/macos . The code itself wasn't touched.

If you get those commits into your PR, I'll happily merge the PR ;-)

[smokris]

OK, I updated the commit messages to match yours, and squashed/dropped the debug commits.

The Travis CI build failure seems to be an issue on their end

[steveschnepp]

Yep, restarting the build made it pass.

[smokris]

Great! Thanks.

[steveschnepp]

note, i started to port plugins to BSD, but it ended up being a little more complex than anticipated...

[steveschnepp]

have a look at #47