firewall: validate max length of zone name

fw3 sets the maximum length of the zone name to 14 and
ignores zone definitions with too long names.
http://nbd.name/gitweb.cgi?p=firewall3.git;a=blob;f=zones.h;hb=HEAD#l25
http://nbd.name/gitweb.cgi?p=firewall3.git;a=blob;f=zones.c;hb=HEAD#l195

Add a simple validation to ensure that the new zone name is short enough.
This should fix issue #345

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>