> + _doit='/usr/bin/su -m vscan 					-c
> '\''/bin/sh -c "/usr/pkg/bin/freshclam -c 2 -d"'\'''
> + eval /usr/bin/su -m vscan -c ''\''/bin/sh' -c
> '"/usr/pkg/bin/freshclam' -c 2 '-d"'\'''
> ++ /usr/bin/su -m vscan -c '/bin/sh -c "/usr/pkg/bin/freshclam -c 2 -d"'
> /bin/bash: vscan: No such file or directory

What is that bash error for?

> I checked the "su" manpage and this Linux version has no "-m" flag.
> I see mine is part of the 'shadow 4.0.3' package. It is still the case
> in the bleeding-edge branch (meaning upgrade has to be manual).

Your su from shadow at that version should have it since it had it way 
before that version and after. su -m is standard.

Did you try running su -m manually to check?

Your error message above is caused by something else.