net: connection: Move proto2str() to utils.c, rename to net_proto2str()

This function is useful for logging in other parts of network stack,
so allow to reuse it. (Will be used for net_context logging in a
follow-up patch).

Change-Id: I8238874584f8f039c94e5d827265944d97d13f6b
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>