boot0cfg: Also allow a file as a trailing argument

[jlduran]

Summary

Modify the disk check to allow arbitrary files as the trailing argument
instead of requiring a live GEOM disk provider.

This enables modifying a boot0 binary file in-place before flashing it
to a disk via gpart bootcode, or using it directly as an argument to
mkimg's partition specification, as these tools cannot directly adjust
the parameters of the boot0 boot manager.

[jlduran]

I did this modification, mostly to allow using it in conjunction with mkimg (see the next review in the series). I cannot immediately see any downsides of also enabling a file for modification, I also didn't know whom to ask for a review, as boot0cfg's usage is slowly fading away.

[imp]

Ah! This explains the other issue I had in a review I saw before this.

[jlduran]

• Add more checks