Add "srun --attach" support, and a bit better handling of a

client dissapearing.  This required adding a cache of outgoing
messages so that the new srun client can see recent traffic at connection
time.

srun's "--steal" parameter is not yet supported.

Also moved the contents of "struct incoming_client_info" and 
"struct outgoing_fd_info" into the other structures.  It just looks
cleaner.