mode +l: do not accept a limit of 0

RFC1459 must be interpreted as assuming '+l 0' is equivilant to -l.  This
agrees with the interpretation in RFC2811 (section 4.2.9).

Furthermore, original IRCd derivatives do not accept '+l 0'.  Thusly,
I believe strongly that InspIRCd should not either as the behaviour of
'+l 0' is technically undefined.

(ref atheme/atheme#115)