sdjournal: use consistent style for errno check
sdjournal: use consistent style for errno check

When calling systemd C functions we were using a mix of patterns for
variable names and error style checking ("err" vs "r", != 0 vs. < 0).
This standardises on "r < 0" everywhere.

As noted in
https://github.com/coreos/go-systemd/pull/124#discussion_r47586556

Supersedes https://github.com/coreos/go-systemd/pull/131

jonboulle/go-systemd,endocode/go-systemd,glerchundi/go-systemd,glerchundi/go-systemd,coreos/go-systemd,endocode/go-systemd,faiq/go-systemd,jareksm/go-systemd,jareksm/go-systemd,brnv/go-systemd,coreos/go-systemd,brnv/go-systemd,faiq/go-systemd,jonboulle/go-systemd