plainbox:logging: log bugs to a dedicated file

This patch adds a dedicated logger where impossible cases can be logged
as presumed bugs in the program. This is somewhat similar to the BUG_ON
macro from the kernel.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>
"automatic merge of lp:~zkrynicki/checkbox/bug-logger/ by tarmac [r=kissiel,sylvain-pineau][bug=][author=zkrynicki]"