Initial import of RFC 2385 (TCP-MD5) digest support.

Initial import of RFC 2385 (TCP-MD5) digest support.

This is the second of two commits; bring in the userland support to finish.

Teach libipsec and setkey about the tcp-md5 class of security associations,
thus allowing administrators to add per-host keys to the SADB for use by
the tcpsignature_compute() function.

Document that a single SPI must be used until such time as the code which
adds support to the SPD to specify flows for tcp-md5 treatment is suitable
for production.

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