exchange.m4: Rename X-Distorted-... headers in messages from outside.

They're quite possibly misleading.  I don't think there's much harm
which can be done by adding extra X-Distorted-Warning headers, but
certainly we don't want anyone confusing things by adding their own
X-Distorted-SpamAssassin-... headers.  (That won't affect the server's
assessment of spamminess in any obvious way, but the purpose of the
header is to give user filters something to act on, so it's important
that they use the true header rather than the wrong one.)