SystemD installation (#1445)

* First implementation; older systemd is not yet supported; SysV is not supported at all

* small correction

* more documentation; manual testing confirms that Athens is working as a SystemD service

* documentation updates

* removed differences from author/master branch

Makefile

@@ -9,9 +9,13 @@ endif
 build: ## build the athens proxy
 	cd cmd/proxy && go build
 
+.PHONY: build-ver
 build-ver: ## build the athens proxy with version number
 	GO111MODULE=on CGO_ENABLED=0 GOPROXY="https://proxy.golang.org" go build -ldflags "-X github.com/gomods/athens/pkg/build.version=$(VERSION) -X github.com/gomods/athens/pkg/build.buildDate=$(DATE)" -o athens ./cmd/proxy
 
+athens:
+	$(MAKE) build-ver
+
 # The build-image step creates a docker image that has all the tools required
 # to perform some CI build steps, instead of relying on them being installed locally
 .PHONY: build-image
@@ -110,6 +114,10 @@ down:
 dev-teardown:
 	docker-compose -p athensdev down -v
 
+.PHONY: clean
+clean: ## delete all locally-built artefacts (not including docker images)
+	rm -f athens cmd/proxy/proxy
+
 .PHONY: help
 help: ## display help page
 	@grep -E '^[a-zA-Z_-]+:.*?## .*$$' $(MAKEFILE_LIST) | sort | awk 'BEGIN {FS = ":.*?## "}; {printf "\033[36m%-30s\033[0m %s\n", $$1, $$2}'