[SA-exim] disable for local users

[jvanasco at mastersofbranding.com]

not to sound inept, although i am...

is there a way to not scan mail for outgoing smtp?

i'd imagine it would be some formula in SAEximRunCond -- but i haven't 
a clue what kind of expansion to put in there (as stuff would not be 
from 127.0.0.1)

would this be an sa-exim thing, or would it be some expansion in exim 
to disably local_scan?