MyISAM repair-by-sort buffer cannot be > 4 GB even on 64-bit architectures

Description:
This applies to both server's myisam_sort_buffer_size and myisamchk
--sort_buffer_size. Even though both options are declared as GET_ULONG
which theoretically allows them to be greater than 4 GB on 64-bit
architectures (with 64-bit Windows being an exception), their value is
eventually cast to uint in _create_index_by_sort().

As a result, a wrap-around value is actually used as a buffer size. When
the wrap-around value is 0, _create_index_by_sort() fails with a
spurious "myisam_sort_buffer_size is too small" error.

How to repeat:
Try using myisam_sort_buffer_size=4G or "myisamchk -r --sort_buffer_size=4G" on a 64-bit
machine.

Suggested fix:
Remove casts to uint in _create_index_by_sort() and calls to it. Better yet, declare the
options as GET_ULL with SIZE_T_MAX as their max allowed value, and change all relevant
variables from ulong to ulonglong to fix 64-bit Windows as well.