confile: add lxc.console.size

confile: add lxc.console.size

lxc.console.size regulates the size of the console log file. This is intended
to replace lxc.console.buffer.logfile. The current semantics are:

- if lxc.console.size is not set:
  - no limit is placed on the size of the log file
- if lxc.console.size is set:
  - if lxc.console.rotate is set and the next write would exceed the limit:
    - write as much as possible into the old log file
    - rotate the log file
    - write as much as posible into the new log file
    - discard remaining bytes (scenario shouldn't be possible in normal
      circumstances)
  - if lxc.console.rotate is not set and the next write would exceed the limit:
    - keep overwriting the current log file

To make the log file a mirror of the in-memory ringbuffer simply set:
lxc.console.buffer.size == lxc.console.size.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>