We fix a bad_bufptr() failure from a previous commit. We now limit
the allowed characters to those that will definitely be safe
in a shell. Some scripts/programs may use arbitrary environment
variables without escaping correctly - that could be a problem
in a restricted environment.
The current allowed set is a-z A-Z 0-9 .,_-+@
This also adds a test for SSH_PUBKEYINFO, by default it only runs
under github actions (or "act -j build").
