Fix regression introduced by push:

3923 Ole John Aske	2012-05-23
      This is the improved 'save_mem.patch' from Mikael R. patch set

That push tried to save memory used for interthread communication buffers
by not allocated buffers between those threads which was assumed to not communicate.
LDM <-> LDM communication was one of the places where such buffers where removed.

However, the BACKUP-block is part of the LDM thread, and during backup the 
first BACKUP instance in LDM thread#1 will act as client/coordinator. Thus
it *will* communicate with other LDM threads!

This fix introduce special handling of LDM thread#1, and 'opens up' communication
between that thread and other LDM threads. Instead of writing a completely new
testcase to cover this fix, I have introduced 'ndb_restore_misc.cnf' which overrides
'MaxNoOfExecutionThreads=4' - Such that at least one of the existing backup-restore
tests are run with multiple LDM threads.

If the BACKUP is later made full multithreaded, the changes to mt.cpp in
this patch should be reverted.