rc: make dump/restore more resilient

Problem: the rc1 flux-restore(1) command can fail if a dumped
value exceeds the maximum content cache blob size.

Call flux-restore(1) in rc1 with the --size-limit=104857600 (100mb)
so that KVS keys exceeding that limit are not restored.  A message
is logged to stderr.

Fixes #5245

Author: garlick

etc/rc1

@@ -47,7 +47,7 @@ if test $RANK -eq 0; then
             flux module load ${backingmod} truncate
         fi
         echo "restoring content from ${dumpfile}"
-        flux restore --quiet --checkpoint ${dumpfile}
+        flux restore --quiet --checkpoint --size-limit=104857600 ${dumpfile}
         if test -n "${dumplink}"; then
             rm -f ${dumplink}
         fi