Don't send XEP-0280 v1 copies back to sender

An earlier version of XEP-0280 specified the <received/> and <sent/>
tags to be siblings of the <forwarded/> element, whereas the current
version mandates them to be parents of <forwarded/>.  The mod_carboncopy
module supports both variants.  However, the check that makes sure
clients won't receive a copy of the messages they sent didn't work for
the old-style schema.  This is now fixed.
Let is_carbon_copy/1 recognize <received/> carbons

The mod_carboncopy:is_carbon_copy/1 function now returns true not only
for <sent/>, but also for <received/> carbon copies.
Simplify mod_carboncopy:check_and_forward/4

Use the existing is_carbon_copy/1 function, and combine multiple case
clauses into a single one.
Merge pull request #229 from weiss/no-carbons-to-sender

XEP-0280: Don't send v1 carbon copies back to the sender