mmwtex.c: boolean-style evaluation in conditionals (+ linting)

[benjub]

Found some inconsistencies while on #160, which I signal as TODOs in this PR. I also made some simplifications (mainly, evaluate boolean-like variables as bools) and linting, all in mmwtex.c.

[benjub]

@digama0 : following approval by Thierry, can you merge this one ?

[benjub]

I didn't apply the four suggestions changing imperative to indicative since I thought it was the opposite convention, but I'm unsure. I'll let you decide to apply or not before merging.

[digama0]

I took a census of how this is used in the .h file comments on all functions. As expected, there has not historically been any consistency about this, but the indicatives seem to be winning:

• Imperative mood: 73 functions
• Indicative mood: 94 functions
• Noun phrase: 31 functions

Doxygen seems to have thrown its opinion into the ring slightly with the \returns keyword. My preference is to standardize on indicative mood, perhaps it should be added as an additional element to the "code cleanup" issue?

[benjub]

perhaps it should be added as an additional element to the "code cleanup" issue?

Done. (Add suggestions applied.)