luci-app-ksmbd: add UI for multiple interfaces

/etc/config/ksmbd supports multiple interfaces (in config globals->
option interface) but the current UI only supports selecting a single
one. This patch adds support for selecting multiple interfaces, based on
how luci-app-samba4 does it.

Closes: #6620

Suggested-by: @AndroKev
Signed-off-by: Rani Hod <rani.hod@gmail.com>