Introduce USE_SOFTRAID to allow testing on vnd(4) directly

Introduce USE_SOFTRAID to allow testing on vnd(4) directly

This is mostly for architectures which don't have softraid(4) support
in installboot(8), but should also be useful in general to test different
setups and thus increase coverage.

Default to root on softraid on vnd on all architectures that support it:
amd64, arm64, i386 and sparc64;  use root on vnd on others (yet to come).