luci-mod-system: extend dropbear listen options

luci-mod-system: extend dropbear listen options

dropbear.init behaviours:

Interface: For each interface, collect all its IPs,
up to 10 endpoints total, and add `-p <addr:port>` for
each; if no interface was given, fall back to a plain
`-p <port>` (listen on all interfaces and IPs). It binds
to addresses, not the device. It collects all IPv4, and
all IPv6 - ULA, but not LL - and binds to those. Ex:

dropbear ... -p 192.168.1.1:22 -p 192.168.1.2:22 -p 192.168.1.3:22 ...

DirectInterface: A single logical interface name; the
first one is used (if multiple are given via `list`).
Runs dropbear with `-l <device> -p <port>`, i.e., it binds
directly to the layer 2 network device rather than
specific IPs. Ex:

dropbear ... -l br-lan -p 22 ...

Closes openwrt/packages#25555
Closes #7484

Signed-off-by: Paul Donald <newtwen+github@gmail.com>