Update rules for logical/control variables

[mkavulich]

Description

Update the rules for logical and control variables to distinguish what prefixes are allowed and when they should be used, based on the proposal in Issue #101. Also update a few standard names based on these rules.

This PR would supersede #149

Issues

Resolves #101

[mkavulich]

@nusbaume When you get a chance, can you look at this and see if it's a good substitute for your #149?

[climbfuji]

I am ok with this. There is always going to be some ambiguity what to use, but hopefully the rules PLUS existing names in the dictionary will help.

[nusbaume]

Thanks for the PR @mkavulich! I'm happy with this replacing my previous PR.

With regards to the standard names, was there a reason why flag_for_ was chosen over is_? I personally find is_ to be clearer, especially when do_ is also allowed (as one is for a given state or condition, while the other is to indicate a specific action, and both make it clear as to what a .true. value means). However I could certainly be convinced otherwise if people have reasons they prefer flag_for_. Thanks!