Add systemd unit config checker

[holomorph]

adds a checker and entry in languages.rst for a systemd unit config checker using systemd-analyze. see #1135

[CLAassistant]

All committers have signed the CLA.

[manuel-uberti]

Thanks for the PR. :-)

[swsnr]

@holomorph Please sign the CLA. We're not able to accept your contribution otherwise.

[swsnr]

Please squash your commits and add Fixes GH-1135 to the commit message so that Github automatically closes that issue if we merge this pull request.

Thanks :)

[holomorph]

boom

[swsnr]

Could you just add a entry to the "New syntax checkers" section of CHANGES.rst?

And might we ask you to write an integration test for the syntax checkers in test/flycheck-test.el? Starting from the end of the file there are tests for the other syntax checkers as well, which serve as an example. Any simple test would suffice.

[holomorph]

@lunaryorn done

[swsnr]

LGTM

[swsnr]

Could you turn this into a "named" link? e.g.

`systemd-analyze`_

.. _systemd-analyze: https://…

It's easier to read in plain text form that way.

Don't forget to refill the paragraph afterwards with M-q.

[holomorph]

@lunaryorn done

[swsnr]

@holomorph Thanks a lot, merged!

[cpitclaudel]

@holomorph Congrats on this contribution!

[manuel-uberti]

@holomorph great stuff, thanks!