Support changing packet count on command line to unlimited

[Michael Clark]

Currently ping takes the last value supplied by '-c x' as the number of ECHO_REQUEST packets to send. If '-c' is not specified it defaults to 0 (false), and has no upper limit on ECHO_REQUEST packets sent. However, once a value for '-c' has been specified on the command line, such as in a bash alias, the value cannot become unlimited due to line 110 of ping_common.c - which checks for 'npackets <= 0'.

Could we change this to 'npackets < 0', to allow later options to change this back to the unlimited option?

Ubuntu 10.4, iputils package up to date as of today.