Implement new configuration option "MaxPenaltyTime" (#251)

Implement new configuration option "MaxPenaltyTime" (#251)

This option configures the maximum penalty time increase in seconds, per
penalty event. Set to -1 for no limit (the default), 0 to disable
penalties altogether. ngIRCd doesn't use penalty increases higher than 2
seconds during normal operation, so values higher than 1 rarely make
sense.

Disabling (or reducing) penalties can greatly speed up "make check" runs
for example, see below, but are mostly a debugging feature and normally
not meant to be used on production systems!

Some example timings running "make check" from my macOS workstation:

- MaxPenaltyTime not set: 4:41,79s
- "MaxPenaltyTime = 1":   3:14,71s
- "MaxPenaltyTime = 0":     25,46s

Closes #249.