campaignd: Emit debug messages when writing configuration to disk
campaignd: Write config to disk even if the clock is running backwards
campaignd: Fix abs() value truncation warning on clang 3.5
campaignd: Use a POSIX monotonic clock for scheduling config commits

Only available on Linux and probably some BSD systems. For other systems
we issue a #warning and use time() instead (which will cause the
compiler to quit with -Werror, but that's the whole point).