Refactor tcopy(1) and teach it about SIMH-TAPFILE format

[phk]

Summary

This is a refactoring of tcopy(1) to make it (also) understand SIMH-TAPFILE format.

I think I have retained all previous behaviour correctly.

The EXAMPLES section show a couple of the things one can use the SIMH-TAPFILE support for.

[imp]

i'm not a simh-tapefile expert, so I've not verified that this implements that standard.
I do like the cleanup generally and have only a few questions / suggestions.

usr.bin/tcopy/tcopy.c

143

the above looks like it should be its own function, just called from here since it handles a completely different case than parsing the SIMH-TAPEFILE format.

175

This code deals poorly with truncated return results, though it would be rare to get partial reads AND not be at EOF, so it just affects the quality of the error messages.

203

Same comment as tape_dev_read

235

Here the order is reversed from tape_dev_{read,write}. Is that intentional?

[ziaee]

May we omit the prompt characters in the examples section of tcopy.1 for copy paste?

usr.bin/tcopy/tcopy.1

93

If we mark it up as Pa, then people can find this page with apropos Pa=sa

112

123

Should we use the opensimh repo instead because of the licensing fiasco?

[phk]

Closing this, continuation in https://reviews.freebsd.org/D51568