re-work -debug switch handling

- make -debug log every debugging information again (even all categories)
- remove unneeded fDebug checks in front of LogPrint(), as that check is
  done in LogPrintf() when category is != NULL (true for LogPrint() calls
- remove fDebug ONLY in code which is NOT performance-critical
- harmonize addrman category name
- deprecate -debugnet usage, should be used via -debug=net and remove the
  corresponding global