Add $OpenBSD$ markers to xmss files to help keep synced with portable.
ok djm@.

Add experimental support for PQC XMSS keys (Extended Hash-Based Signatures)
The code is not compiled in by default (see WITH_XMSS in Makefile.inc)
Joint work with stefan-lukas_gazdag at genua.eu
See https://tools.ietf.org/html/draft-irtf-cfrg-xmss-hash-based-signatures-12
ok djm@