libnet: Compile with -Wextra

-Wextra enables a bunch of rather useful checks which this fixes.

This also fixes unused parameters warning by passing meaningful value and
doing sanity checks.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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Changes:
v2:
* updated commit log about using AF_INET/etc
* replaced cast to int with size_t in pxelinux_load_cfg
* added (alen == 0) in ping()