lxcpp: Initial implementation of the start API call

lxcpp: Initial implementation of the start API call

[Feature]       Initial implementation of the start API call.
[Verification]  Build, install, run tests

Significant changes related to the start API call:
- Start command that daemonizes and execs guard binary
- Guard process implementation that execs the container's init

Additional changes in this commit supporting the Start API call:
- API extension to Channel class, support close on exec and be able
  to survive (as a FD) exec() call
- set close on exec in persistent file logger
- new logger helper to setup the logger
- add pid to the log format

Change-Id: I2d9648e2a861add2aa1bd1d66587bff2a109cc9c