This patch adds NATT_EXTRA_PATCHES=natt.diff and enables only UDP encapsulation defined in RFC3948.

The natt.diff patch contains the following changes:

• added support for SADB_X_EXT_NAT_T_OAI and SADB_X_EXT_NAT_T_OAR PF_KEY messages;
• used NAT address instead of original for SAs created by racoon;
• NAT-T keep-alives now sends only by NATed host.

Tested with 11.0-STABLE after projects/ipsec merge.

PR: 217131
Submitted by: Andrey V. Elsukov
Approved by: VANHULLEBUS Yvan (maintainer timeout, 2 months), vsevolod (mentor)