Remove xform_ipip.c and code related to XF_IP4.

The only thing is used from this code is ipip_output() function, that does
IPIP encapsulation. Other parts of XF_IP4 code were removed in r275133.
Also it isn't possible to configure the use of XF_IP4, nor from userland
via setkey(8), nor from the kernel.

Simplify the ipip_output() function and rename it to ipsec_encap().

• move IP_DF handling from ipsec4_process_packet() into ipsec_encap();
• since ipsec_encap() called from ipsec[64]_process_packet(), it is safe to assume that mbuf is contiguous at least to IP header for used IP version. Remove all unneeded m_pullup(), m_copydata and related checks.
• use V_ip_defttl and V_ip6_defhlim for outer headers;
• use V_ip4_ipsec_ecn and V_ip6_ipsec_ecn for outer headers;
• move all diagnostic messages to the ipsec_encap() callers;
• simplify handling of ipsec_encap() results: if it returns non zero value, print diagnostic message and free mbuf.
• some style(9) fixes.

Differential Revision: https://reviews.freebsd.org/D2303
Reviewed by: glebius
Sponsored by: Yandex LLC