Add kernel option IPSEC_SUPPORT.

It is supposed to be used in kernel that supports IPsec as kernel module.
Currently we don't have ability to unregister network domain(9), thus to
have loadable IPsec, we need PF_KEY is build in the kernel.

There will be three options to control IPsec build:

• options IPSEC: all IPsec related code is build in the kernel.
• options TCP_SIGNATURE: TCP-MD5 support is build in the kernel.
• options IPSEC_SUPPORT: PF_KEY support is build in the kernel and IPsec consumer code contains runtime checks for presence of IPsec modules.

TCP_SIGNATURE support is also planned to be implemented as kernel module.
If kernel has IPSEC_SUPPORT option, but has not TCP_SIGNATURE option,
it will be possible to load TCP_SIGNATURE as module.
If kernel has IPSEC option, but has not TCP_SIGNATURE option - TCP_SIGNATURE
support will not be available.
If both IPSEC and IPSEC_SUPPORT are enabled, but TCP_SIGNATURE is not -
TCP_SIGNATURE support would be available as kernel module.