Revert local changes made to make zstd(1) frontend behave like gzip(1) and friends

This change was made to allow zstd(1) to be a dropin replacement for gzip(1) and
friends, allowing easy integration, in particular with newsyslog(8). At the
price of having a zstd(1) command which by default behaves differently than what
upstream default, confusing users.

newsyslog(8) has been adapted to now be more flexible in what it accepts as
compression program, so we can switch back zstd(1) to its default behaviour

Reported by: many