rfc7: add std for variable names and typedefs

[garlick]

There was some discussion in PR flux-framework/flux-core#205 about names for function pointer typedefs.

I'm proposing that we add SHOULD-level guidelines for

• lower case C variable names
• upper case C preprocessor macros
• non-function typedefs: lower case ending in _t
• function typedefs: lower case ending in _f and defined as function pointers
• use typedef pointer to incomplete type to publicly define abstract data types

[garlick]

Oh, interesting. I don't especially have an opinion, so I'll force an update of this PR without the break long lines patch, and my patch conforming to the style of the original doc.