samba: add file/interface reload triggers & filter interfaces

samba: add file/interface reload triggers & filter interfaces

* Only parse interfaces that are up during init_config (as the
  script depends on this to determine the proper IP/subnet range)
* Add reload interface triggers for samba-designated interfaces
* Force full service restart upon config change to ensure Samba
  binds to new interfaces (sending HUP signal doesn't work)
* Rename "interface" variable to "samba_iface" and move into
  global scope

Needed to fix Samba connectivity for clients connecting from a
different LAN subnet (e.g. pseudobridge configurations) due to the
'bind interfaces only' setting.

Signed-off-by: Conn O'Griofa <connogriofa@gmail.com>