Manual upgrade doesn't use dd(1) anymore

Use another sentinel file to track the progress of the upgrade.
Add final version of 9.2.1.8 Errata. The HTML needs to be added separately.
Use a resource mixin for Upgrade
Add API endpoint to perform a manual update

Ticket:	#5334
Work towards trains, also release notes (et al) for manifests.
Do the right thing in save_rrds.sh

Ticket:	#6208
Do not mix tab and spaces
Unbreak if nfs_security is empty
Call form done method from API

Ticket:	#6218
Make WebDAV form PEP8 compliant and allow translation of error message
Use secure_path in sudoers to fix a local privilege escalation bug

Our system mail handler runs as root using sudo, however the code
responsible for getting the software name (/etc/rc.freenas) runs
commands using relative path, which is vulnerable to PATH changes.

This change makes sure PATH environment var is reset to a sane default
prior to /etc/find_alias_for_smtplib.py execution.

Reported by:	William Söderberg <william.soderberg cuebid se>
Copy release notes (et al) to be processed on the remote end.
Add rc.conf info.
More notes tweaking.
Minor changes to Network chapter.
Save private key when generating a CSR
Start review of Storage chapter.
Major logic error:  if the manifest suggests updates are
available, but they're not, it would fail out when it
couldn't find the update file.
Fix unintentional linebreak
Use the package checksum.