Bug#1080322: latexmk: add -parallel=N option

Here's information on the bugs I found when parallel mode is used:

1. When I use the option -g to latexmk to force all rules to be run at least 
once, the parallelization code vetos the running of the custom dependencies 
because the built file is "already up-to-date (built by concurrent process)". 
The code is using a test that is wrong, even though it appears natural.

2. More importantly, when I use parallel mode on my colleague's document that I 
mentioned in my previous e-mail, it keeps trying to rerun all the custom 
dependencies until it hits the limit of the maximum number of allowed runs.   I 
conjecture from the behavior that there is something going wrong in the use of 
dependency information.

If you are interested, I can send MWEs.

John