ping: add -p to specify data pattern

function                                             old     new   delta
common_ping_main                                    1788    1831     +43
sendping6                                             56      82     +26
sendping4                                             82     108     +26
packed_usage                                       29959   29938     -21
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(add/remove: 0/0 grow/shrink: 3/1 up/down: 95/-21)             Total: 74 bytes

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>