XEP-0198: Check whether routed packets are stanzas

Only stanzas are subject to stream management, so when XEP-0198 support
is enabled, we must distinguish them from non-stanza elements.  This
commit adds a send_packet/2 function that can be used in place of
send_stanza/2 or send_element/2 whenever a packet is delivered that
might or might not be a stanza.
Merge branch 'check-packet-type' of git://github.com/weiss/ejabberd into weiss-check-packet-type

Conflicts:
	src/ejabberd_c2s.erl
Merge branch 'weiss-check-packet-type' into 3