Backport from 6.0.14 to 5.6.0

Original code from Guilhem Bichot

Backport from 6.0.14 to 5.6.0

Original code from Sergei Golubchik

Backport from 6.0.14 to 5.6.0

Original code from Sergei Golubchik

  In fact this crashes in normal (not embedded) run also.
  The problem is in the memory mapping. Handling the ha_myisammrg::extra(MMAP)
  the MERGE engine tries to mmap all the tables it unites.
  Though some can be empty and then in the mi_dynmap_file()
  we call the my_mmap(0). Normally this call returns MAP_FAILED,
  but not on FreeBSD. There it returns like a 'normal' value,
  and after the consequitive munmap systems gets unstable and
  crashes on some system call later.

per-file comments:
  storage/myisam/mi_dynrec.c
Bug #47139      Test "merge" crashes in "embedded" run
    don't try to mmap zero-length area, just return at once.

   The additional patch. That 'loadxml.test' failure was actually about our testing system,
   not the code.
   Firstly we need a new mysqltest command, wich i called 'send_eval'. So the expression
   can be evaluated, then started in a parallel thread. We only have separane 'send' and
   'eval' commands at the moment.
   Then we need to add the waiting code after the 'KILL' to our test, so the thread will be killed
   before the test goes further. The present 'reap' command doesn't handle the killed threads
   well.
      
per-file comments:
  client/mysqltest.cc
Bug#42520      killing load .. infile Assertion failed: ! is_set(), file .\sql_error.cc, line 8
    The 'send_eval' command implemented.

  mysql-test/r/loadxml.result
Bug#42520      killing load .. infile Assertion failed: ! is_set(), file .\sql_error.cc, line 8
   test result updated.

  mysql-test/t/loadxml.test
Bug#42520      killing load .. infile Assertion failed: ! is_set(), file .\sql_error.cc, line 8
   test case added.

(revision-id: kristofer.pettersson@sun.com-20091019074333-g2ces0lo4c2ejar7),
because it broke the tree and was reverted later.

Fix partition_column.test.