Instead of always returning a `strdup()`'ed version, maybe return NULL and
handle this in the calling code.
This is still not true for `plugins_pre_chat_message_display()`, where we
return the passed `messag` in case there are no plugins, since this would
require a bigger refactor of more parts.
This also
* merges the implementation of `sv_ev_incoming_private_message()` and
`sv_ev_delayed_private_message()`, since they differed only by a single
line.
* untangles the `#ifdef` mess in `cl_ev_send_muc_msg_corrected()`.
Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>
83a4165d92