BUG#13987 Cluster: Loss of data nodes can cause high CPU usage from ndb_mgmd

fix the actual problem (getting incomplete line of data), introduced with previous
improvement.

also add list sessions, get session and get session id to mgmapi to allow
the implementation of a test case for this exact problem.


ndb/include/mgmapi/mgmapi.h:
  Add internal ndb_mgm_get_fd for use in test case
ndb/include/mgmapi/mgmapi_debug.h:
  Add internal get_session_id and get_session
ndb/include/util/InputStream.hpp:
  - fix warning when building with gcc 4
  - add mutex to be UNLOCKED when blocking (e.g. select(2))
    - this means we can list sessions in a threadsafe way
  - add this weird startover member to SocketInputStream
  	- this helps work out if we've read a newline yet and should start inserting
  	  into buffer from the start
ndb/include/util/Parser.hpp:
  add mutex to context to pass down to SocketInputStream
ndb/include/util/socket_io.h:
  readln_socket accepts mutex to UNLOCK around select(2)
ndb/src/common/util/InputStream.cpp:
  remove evil, add more.
  
  As
ndb/src/common/util/Parser.cpp:
  set mutex for passing down to InputStream to unlock on select(2).
  
  change way detecting of NoLine
ndb/src/common/util/socket_io.cpp:
  unlock a mutex around select so that we can nicely do thread safe
  listing of sessions.
  
  Always retrieve data from the OS so that we instantly get EOF on disconnect
  and don't end up spinning looking for a newline.
ndb/src/mgmapi/mgmapi.cpp:
  add internal ndb_mgm_get_fd() for internal testing
  
  internal/debug:
  ndb_mgm_get_session_id
  
  ndb_mgm_get_session
ndb/src/mgmsrv/Services.cpp:
  Add list sessions, get session id and get session.
  
  introduce a session mutex
ndb/src/mgmsrv/Services.hpp:
  Add list and get session.
  
  Add session_id to MgmApiSession.
ndb/test/ndbapi/testMgm.cpp:
  Add test for MgmApiSession disconnection (mgmd at 100%)