factory: symlink man config file

If this config is absent from /etc/ man fails:

$ man systemd
man: can't open the manpath configuration file /etc/manpath.config
$

Author: bluca

mkosi.extra/usr/lib/tmpfiles.d/etc.conf

@@ -65,3 +65,5 @@ L? /etc/alternatives
 L? /etc/PackageKit
 # ModemManager needds its dbus policy file
 L? /etc/dbus1/systemd.d/org.freedesktop.ModemManager1.conf
+# man fails without this in /etc/
+L? /etc/manpath.config