config: Allow overriding default values with values from a config file for --generate-config.

You can now do the following:

./app --generate-config --config-file=overrides.ini

And any value in overrides.ini will be placed into the generated file
as the config's value. If the value in the config file matches the
default value, it will be added to the config file as normal.
QEV_*_TO_USEC: use `ll` suffix to prevent overflows
pc.in: Only use parent of include dir

The header should be included with <quick-event/qev.h>.