Add missing fclose after tape format detection.

[atsampson]

Without this, doing something like:

devmount mta0 /tmp/foo
devunmount mta0

(i.e. without specifying fmt=bar to the devmount) will leak a FILE * and file descriptor. It looks like df isn't used after this point in the function, aside from the error cleanup code at the bottom...

[larsbrinkhoff]

Ping @atsampson and @Rhialto. This pull request seems stuck in limbo. Can we push it forward somehow?

[Rhialto]

I suppose I could merge as-is, and then do the changes I was thinking about myself.