Add configurable log size limit and retention count

For the following log files:
  hungup.log
  error.log
  crash.log
  hack.log
  spam.log
  guru.log

... they can be limited in size, in bytes, by the sysop, along with a maximum retention limit (number of *.#.log files). Configured in SCFG->System->Advanced.

By default, the limit and keep numbers are 0, so "unlimited" (same behavior as before). Once sufficiently tested, I'll change the default in a stock main.cnf file to use a sensible limit (e.g. 10 log files of 10MB each, something like that).