data and index files

It was possible if DATA/INDEX DIRECTORY is pointing to
symlinked MySQL data home directory.

Do not allow to drop data/index files implicitly symlinked
to data home directory. For such tables remove symlink only.

mysql-test/r/symlink.result:
  A test case for BUG#40980.
mysql-test/t/symlink.test:
  A test case for BUG#40980.
storage/myisam/mi_delete_table.c:
  Do not allow to drop data/index files implicitly symlinked
  to data home directory. For such tables remove symlink only.