MDEV-17470 Orphan temporary files after interrupted ALTER cause InnoDB: Operating system error number 17 and eventual fatal error 71

Orphan #sql* tables may remain after ALTER TABLE
was interrupted by timeout or KILL or client disconnect.

This is a regression caused by MDEV-16515.

Similar to temporary tables (MDEV-16647), we had better ignore the
KILL when dropping the original table in the final part of ALTER TABLE.

Closes #1020