src/utils_complain.[ch]: Reintroduced an improved version of the complain mechanism.

src/utils_complain.[ch]: Reintroduced an improved version of the complain mechanism.

This has been asked for a couple of times and since I need it now, here it
goes... ;-) I've done the following modifications to Florian's original
version:

 * Renamed "plugin_complain" to "c_complain".
 * Renamed "plugin_relief" to "c_release", thus using two verbs which is
   more consistent imho.
 * Added "c_complain_once" to report a complaint only once.
 * Use timestamps instead of counting multiples of "interval_g" to
   determine when to report again. Thus, multiple calls to the complain-
   functions in one interval are allowed.

Signed-off-by: Sebastian Harl <sh@tokkee.org>
Signed-off-by: Florian Forster <octo@huhu.verplant.org>