loader: Simplify build a little

Confine -DDISK_DEBUG to biosdisc.c, the only file it affects.
Use modern variable arrays instead of alloca and add a sanity
size minimum for biospnp nodes. These nodes are tiny enough that
we needn't do a malloc/free pair: the stack is fine.

Sponsored by: Netflix
Reviewed by: tsoome
Differential Revision: https://reviews.freebsd.org/D43914