Sanity checks when opening MyISAM files


Docs/manual.texi:
  Added information about Borland c++
myisam/mi_check.c:
  Cleanup
myisam/mi_open.c:
  Added sanity checking
myisam/myisamchk.c:
  Better error messages
mysys/my_chsize.c:
  Cleanup
mysys/my_seek.c:
  Changed debug message
mysys/my_write.c:
  Fixed test when exeeding file quota