Ken's last note led me to look at magtape error handling, and it was
not right. This has resulted in a lot of code changes in sigma_mt.c, but
the changes are mostly mechanical. Instead of trying to be clever and
do channel handling in mt_map_err or at the bottom of the mt_svc
routine, every magtape command now does its own error handling.
This makes the code repetitive, but I hope it is much clearer as a result.