expose fewer internals in packages, for easier software reuse

- prometheus is now behind an interface, they aren't dependencies for the
  reusable components anymore.
- some dependencies have been inverted: instead of packages importing a main
  package to get configuration, the main package now sets configuration in
  these packages. that means fewer internals are pulled in.
- some functions now have new parameters for values that were retrieved from
  package "mox-".

moxio/tlsinfo.go

@@ -0,0 +1,29 @@
+package moxio
+
+import (
+	"crypto/tls"
+	"fmt"
+)
+
+// TLSInfo returns human-readable strings about the TLS connection, for use in
+// logging.
+func TLSInfo(conn *tls.Conn) (version, ciphersuite string) {
+	st := conn.ConnectionState()
+
+	versions := map[uint16]string{
+		tls.VersionTLS10: "TLS1.0",
+		tls.VersionTLS11: "TLS1.1",
+		tls.VersionTLS12: "TLS1.2",
+		tls.VersionTLS13: "TLS1.3",
+	}
+
+	v, ok := versions[st.Version]
+	if ok {
+		version = v
+	} else {
+		version = fmt.Sprintf("TLS %x", st.Version)
+	}
+
+	ciphersuite = tls.CipherSuiteName(st.CipherSuite)
+	return
+}