In the bootblocks, after discovering and opening /bsd.upgrade, fchmod -x

so the file cannot be re-executed upon the next boot.  This provides a
stronger one-shot-upgrade model than the upgrade script's rm /bsd.upgrade.
Now various forms of upgrade failure will reboot into /bsd, which is probably
more recoverable.  Performing fchmod -x depends on (1) use of MI boot.c
(not alpha/macppc/sparc64/sgi/octeon) and (2) "can write blocks" functionality
in the IO layer.  Most architectures have this support now.

Two diagnostics "fchmod a-x %s: failed" and "/bsd.upgrade is not u+x" will
remain in the tree while refinements happen for some of the laggard
architectures.

based upon a discussion florian
tested in snapshots for more than a week without any complaints

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