- Rework the windows implementation in 'create_temp_file' to be
   thread safe by using GetTempFileName instad of fiddling
   with "environ"


mysys/mf_tempfile.c:
  - Update windows implementation  of 'create_temp_file' to use
    GetTempFileName in favor of fiddeling with "environ" in an unsafe way
  - Remove the implementation that is supposed to be used if not
    windows, not mkstemp or tmpnam exists as it not longer compiles it
    can't be used anywhere.
  - Update function comment for 'create_temp_file'
mysys/my_static.c:
  Remove unused variable
mysys/my_static.h:
  Remove unused variable