Add ESP dummy frames support

[aurelien.cazuc.external_stormshield.eu]

Summary

This patch provides support of dummy frames as specified by RFC 4303
Packets with IPPROTO_NONE are silently dropped just before ipsecX_common_input_cb

Test Plan

In manual testing, when generating packets with IPPROTO_NONE as next_header, packets are correctly dropped

[ae]

sys/netipsec/xform_esp.c

629

This indentation does not conform to style(9). I think you can just use

if (lastthree[2] == IPPROTO_NONE)
      goto bad;

the error variable should already be zero.

[aurelien.cazuc.external_stormshield.eu]

Remarks from ae

[aurelien.cazuc.external_stormshield.eu]

Hi @ae ,

Would you prefer commit it yourself or let fabient commit it ?
If you commit it yourself, please mention Stormshield as sponsor.

Thanks