Implement restricted mode for ed

[briandfoy]

Under -r, the ! command is not avialable and ed cannot run shell commands. See #405

[mknos]

In my understanding the restricted-ed mode is not a POSIX-standard feature of ed [1]. In terms of portability, "ed is always there" is truer than "red is always there".
If restricted mode is added here filenames starting in "!" also need invalidating, to prevent reading and writing to shell commands with r ! this and w ! that.

1. https://pubs.opengroup.org/onlinepubs/9799919799/utilities/ed.html