Unix installer: add support for command-line arguments

Command-line arguments replace interactive prompts, which is
better for scripting runs of the installer.
file/untar: fix handling of filename prefix in USTAR format

Combine prefix and filename with a "/".
fix typo in doc implementation
tools doc: fix use of block in an element

Also, adjust @history[] use to make the change more clear in
rendered docs.
Merge branch 'master' of https://github.com/plt/racket into typed/racket/async-channel
add @history[] line for typed/racket/async-channel