sys: move sys/kern/md[45].c to sys/crypto

[fuz]

Summary

Both files are used by kernel and userspace.
Move them to sys/crypto where they belong.

No functional changes intended.

In preparation of D45670.

Test Plan

builds fine

[markj]

stand/libsa/Makefile

191–192

I'd remove this comment too, it's not useful.

[fuz]

Will do. I had to stop with the MD5 changeset as I noticed a performance regression on AMD Zen 4, which I will have to look into in more detail.