WL#315: Merge filesort.cc and myisam/sort.c

One should write a black-box version of filesort.cc that is not depending of
MySQL and put this into mysys.

Then one should change  myisam/sort.cc to use the new filesort interface.

This should be one so that we have only one copy of the sort code.

The code in sql/filesort.cc is newer and more general than myisam/sort.cc, so
one should use this as a base for this.