packet lib: add get_protocol API
packet lib: add get_protocol API

get_protocols returns the list of protocols. This is useful for a
packet including the same protocol multiple times (e.g. tunneling such
GRE). However, it's a rare use case. Instead of
'get_protocols(hoge)[0]', let's do 'get_protocol(hoge)' simply.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

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