Use a function call to change the unit of intermediated interval report columns
Use inline for usage_hint()
Update todo
Column visibility based on involved OSes
Initialize opt.force_unit with INT_MAX
Delete unnecessary returns
Coding style (use underscores in function names)
Move sighandling in front of main function
 Restructure header file
Change select() call timeout back to the old behavior.

With e935625cdb00cdcedbe515307493ca349f56b40d flowgrind's timer management has
changed to achieve a nanosecond precision. The select() call relies on a
timeout which was also affected by the change. This patch changes the timeout
of the select() call back to the old behavior.