...and update man pages where applicable

Open the incomplete download in append mode instead of write mode.
Opening in write mode truncates the existing file, so curl keeps
restarting downloads instead of resuming the incomplete downloads.

Use instance methods to DRY things up, and fix a bug discovered in
caskroom/homebrew-cask#26067 where Casks with language blocks would
always fail audit.

- Stop supporting archaic "user-repo/token" syntax
- Move regex for parsing tapped Cask token to tap_constants

fixes 2 issues: error when username has a number, correct url for repos not on github

Handle the case when an argument passed to `brew cask` is both the name
of a Cask and a non-Cask file in the current working directory.

Fixes https://github.com/caskroom/homebrew-cask/issues/25611.

There is an existing `SystemCommand.run` that executes command with
`sudo`.  Use that instead of yet another custom wrapper.