dovecot: auto-detect sockets in /run/dovecot2

NixOS uses this path, and it seems generic enough to be worth adding
here.

The `/var/run` paths should probably also be updated to `/run`, but I'm
not going to risk breaking anything :)

Amended-by: Alberto Bertogli <albertito@blitiri.com.ar>
  Adjusted comments.

internal/dovecot/dovecot.go

@@ -34,11 +34,21 @@ var (
 )
 
 var defaultUserdbPaths = []string{
+	// NixOS.
+	"/run/dovecot2/auth-chasquid-userdb",
+	"/run/dovecot2/auth-userdb",
+
+	// Debian, Ubuntu, CentOS.
 	"/var/run/dovecot/auth-chasquid-userdb",
 	"/var/run/dovecot/auth-userdb",
 }
 
 var defaultClientPaths = []string{
+	// NixOS.
+	"/run/dovecot2/auth-chasquid-client",
+	"/run/dovecot2/auth-client",
+
+	// Debian, Ubuntu, CentOS.
 	"/var/run/dovecot/auth-chasquid-client",
 	"/var/run/dovecot/auth-client",
 }