Add support for user-defined interfaces for Samba
This adds a new NVRAM variable, smbd_ifnames, a space-delimited list of
interface names to bind to. It is used by the "interfaces" smb.conf
directive. The variable defaults to br0 (lan_ifname) in cases where
the user has upgraded the FW without resetting NVRAM to defaults (i.e.
smbd_ifnames is unset), or when leaving the GUI form field empty.
Reference: https://www.samba.org/samba/docs/man/manpages-3/smb.conf.5.html
Reference: https://www.samba.org/~tpot/articles/multiple-interfaces.html
Reference: http://www.linksysinfo.org/index.php?threads/attempt-at-patching-smb-custom-vs-default-config-conflict-for-interfaces-option.73024/