ibtop: Improve display

This commit improves the display of ibtop. The display is still
not resizable but looks pretty nice on sufficiently large windows.
ibtop: Add utility timedef_X() functions

This commit adds new functions to compute time differences
from struct timeval instances.
ibtop: Add logging functions

This commit adds new functions for logging and debugging purpose.
The current version always enables debug output and always writes
to a file named "ibtop.log" in the current working directory.
ibtop: Add logging/debugging infos

This commit adds a couple of calls to ibtop_log() and ibtop_debug()
at interesting spots. We measure the time of (some of the)
potentially expensive operations.