Use id_pm as default value for id_pm_head in personal_messages (SMF 1.1 -> ElkArte)

This change emulates the personal message threading scheme
used by SMF 1.1 to 2.0 upgrade script. It simply creates a
separate thread for each message. Without this change, all
imported messages get grouped into the same null message
thread.

Signed-off-by: Noteworthy Software, Inc <online@noteworthysoftware.com>