Add ClassAd attribute passing to DAGMan Vars #3469

Implement this request from Igor:

	Date: Thu, 24 Jan 2013 08:57:03 -0800
	From: Igor Sfiligoi <sfiligoi@fnal.gov>
	To: <htcondor-support@cs.wisc.edu>
	Subject: Re: [htcondor-support 8254] dagman does not allow variables that
	 start with a plus

	On 01/24/2013 08:38 AM, htcondor-support response tracking system wrote:
	> On Thu, Jan 24, 2013 at 10:31:30AM -0600, johnkn wrote:
	>> Dear HTCondor team.
	>>
	>> I just found out that a DAG node cannot be passed a variable that
	>> starts with a plus (+).
	>>
	>> Why?
	>> Looks like dagman just fails while verifying the variable name, even
	>> if this name would be happily accepted by condor_submit -a !
	>>
	>> It would be very useful to add arbitrary attributes that get
	>> propagated in the job classad, without the need to explicitly
	>> reference them in the submit file.
	>>
	>> Can this be considered a bug, and fixed ASAP?
	>>
	>> Thanks,
	>>     Igor
	>
	> Igor, just to be clear:
	>
	> Do you mean that you cannot have a line like
	>
	> VARS +A="bob"
	>
	> in your DAG file?
	Correct.

	Igor

Add test case for using attributes in DAGMan Vars.  Add version history
and update the manual.
Gittrac #3469 review:  clarified the wording about this in the DAGMan
section of the manual, and made a link from the version history to
the relevant section; also clarified the error messages for parsing
problems related to VARS.
Merge remote-tracking branch 'origin/V7_9-gittrac_3469-branch'