Start working on stashing the errno value for every command in the cmd->notes
table.  This will allow modules (such as audit modules for Bug#3807) access
to that errno value, for tracking/logging.
Merge branch 'master' into core-cmd-errno-note
Explicitly set/stash the errno value for a failed command at the point when
PR_ERROR is used on a command.
Merge pull request #25 from proftpd/core-cmd-errno-note

Stash an errno value for all commands, for use by e.g. auditing modules