Do not attempt to write to read-only softraid

Do not attempt to write to read-only softraid

Bootloaders have no write support for softraid volumes, which manifests in,
e.g. /bsd.upgrade not being stripped of its 'x' permission bit to prevent
sysupgrade loops in case of upgrade failure.

Set a no-write flag handled by libsa to bail out early in write calls.
There should be no real behaviour change, writes just fail earlier now.

amd64 BIOS bits first, rest to come.  Crank minor.

"seems sensible" jsing