Previously, only a file with the name `<subcommand-name>` (in the configured
directory) was considered as subcommand completion file for the subcommand with
that name `<subcommand-name>`.

Like bash-completion, fall back to `<subcommand-name>.bash` when the former does
not exist, is not readable or is not a regular file.
Unlike bash-completion, don’t fall back to `_<subcommand-name>`, which is
considered for internal/legacy use only.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>

This script provides bash completions for `remote-forward-credentials` in two
stages.
First, it completes the configured subcommands. Second, if a subcommand-specific
completion file is found, it “delegates” further completions to that.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>