Update net/chrony: enable privilege separation and other minor changes.

• enables privilege separation
• removes the build dependency on asciidoctor
• removes the runtime dependency on makeinfo and readline
• add a runtime dependency on libedit
• do not install the HTML documentation (in favour of man pages)
• update the post-install message (pkg-message) in light of privilege separation
• set the permission of /var/db/chrony to the new "chronyd" user and group

PR: 216737
Submitted by: maintainer
Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D9570