main() is one call into internal/cli (closes #160)

cmd/netwatch-server/main.go parsed the command line, ran the check-cidr
and prepare-data-dir subcommands and built the fx application itself;
the Go style guide puts all of that in internal/ or pkg/. The new
package internal/cli does it, with the same arguments, messages and
exit codes. main() is now os.Exit(cli.Main(Appname, Version,
os.Args[1:], cli.RunServer)): Main takes the function that runs the
server, so its tests call it with one that fails them. Appname and
Version stay in main, where the build sets Version. Each subcommand
writes its error to a writer it is given, and prepare-data-dir also
takes the volume and the user's name, so its own tests give it a
temporary directory and the user they run as.

Model: opus-5-5

internal/cli/export_test.go

@@ -0,0 +1,16 @@
+package cli
+
+import "io"
+
+// CheckCIDR exposes the check-cidr subcommand to the external tests,
+// which read what it writes to stderr.
+func CheckCIDR(stderr io.Writer, cidr string) int {
+	return checkCIDR(stderr, cidr)
+}
+
+// PrepareDataDir exposes the prepare-data-dir subcommand to the
+// external tests, which give it a volume and a user of their own in
+// place of /data and netwatch.
+func PrepareDataDir(stderr io.Writer, volume, dataDir, username string) int {
+	return prepareDataDir(stderr, volume, dataDir, username)
+}