A donor node does a flush tables and then tries to
freeze innodb writes before proceeding with SST.
However, innodb_disallow_writes was missing in xtradb.
Merged 'InnodbFreeze' patch from maria-5.5-galera's to
xtradb. Also, merged some changes missing in innobase's
os0file.cc.

Added a basic test case for innodb_disallow_writes system
variable.