fix(logs/sinks...): platform independent build

factor apart systemd logging for every arch except windows, aiming for a
resolution to https://github.com/coreos/etcd/issues/220

sinks_unix.go - it is assumed that any non windows platform has systemd
sinks_windows.go - the '+build windows' is redundant  since the build
architecture is defined in the file name but is included here for
completeness