incorrect MIN_INTERVAL in pinger.c

as reported at http://www.zabbix.com/documentation/1.8/manual/config/items#comment_66f9462d001f2b8e820a1ca14aa4887f - definition of MIN_INTERVAL in pinger.c is incorrect.

for non-root users fping refuses to run if interval (-p parameter, the one zabbix uses) is less than 20.

this will fail silently in zabbix unless debuglevel 4 is used.

in this case server should refuse to run fping and should insert error information in items table