Handle ^V and slash in TOPS-20 file names

[larsbrinkhoff]

Update read20 to do something clever with file names including characters such as ^V and /.

From what I understand, ^V just quotes the next character so it shouldn't appear in the output. The / character isn't possible in Unix file names so must be translated to something else.

CC @rmaldersoniii for TOPS-20 insights.

[larsbrinkhoff]

Real-life examples:

• kansas:<s.distribution>cm1-7^V/12^V/83-distribution.map.1,s/demo/wormch.qbin
• ps:^V/undeliverable-mail^V/..1,olson/theterm.ida