luci-app-lxc: fix "plain-vanilla" integration, part 2

I've tried to get the lxc app in a more usable state. Tested with mips
and amd64 targets.

* check /etc/config/lxc in controller, not in cbi
* more controller cleanups
* remove unused 'fork_exec' function
* check path before container creation
* check space requirements before container creation
  * support new uci options 'min_space' and 'min_temp',
    default for both is 100000 KB
  * both options are configurable via LuCI CBI template
* write messages to log in case of an error
* validate the container name during creation,
  automatically remove invalid chars
* inform the user that only a stopped container can be destroyed
* add experimental ssl support (untested, disabled by default)

Signed-off-by: Dirk Brenken <dev@brenken.org>