With certain data sets (when compressed record length gets bigger than
uncompressed) myisamchk --unpack may corrupt data file.

Fixed that record length was wrongly restored from compressed table.


myisam/mi_check.c:
  With compressed tables compressed record length may be bigger than
  pack_reclength, thus we may allocate insufficient memory for record
  buffer.
  
  Let single function allocate record buffer, performing needed record
  length calculations.
  
  Still, it is not doable with parallel repair, as it allocates needed
  record buffers at once. For parellel repair added better record length
  calculation.
myisam/mi_open.c:
  When calculating record buffer size, take into account that compressed
  record length may be bigger than uncompressed.
myisam/mi_packrec.c:
  With certain data set share->max_pack_length (compressed record length)
  may be bigger than share->base.pack_reclength (packed record length).
  
  set_if_bigger(pack_reclength, max_pack_length) in this case causes
  myisamchk --unpack to write extra garbage, whereas pack_reclength
  remains the same in new index file. As a result we get unreadable
  table.
myisam/myisamchk.c:
  With compressed tables compressed record length may be bigger than
  pack_reclength, thus we may allocate insufficient memory for record
  buffer.
  
  Let single function allocate record buffer, performing needed record
  length calculations.
mysql-test/mysql-test-run.pl:
  Environment variables to execute myisamchk and myisampack.
mysql-test/r/myisampack.result:
  New BitKeeper file ``mysql-test/r/myisampack.result''
mysql-test/t/myisampack.test:
  New BitKeeper file ``mysql-test/t/myisampack.test''