In sbbs_t::getsmsg(), don't lock/read/write/unlock every single node

record in the node.dab - only the node(s) that have the requested user
number.
In sbbs_t::getsmsg() and ::getsmsg(), strip all invalid Ctrl-A attribute
codes (e.g. clear screen, pause, etc.) since short-messages (telegrams)
can come from external sources.