replace 'cond and one or tother' idiom with 'one if cond or tother'

[daira]

In Python pre-2.5, the nearest equivalent to C's cond ? one : tother was the idiom cond and one or tother. In Python 2.5+, a more precise equivalent is one if cond else tother.

The former is error-prone: it evaluates to tother when one is falsy, which is the wrong thing. Since Tahoe-LAFS requires at least Python 2.6, we can always use the latter.

("t'other" is Northern English dialect for "the other", if you were wondering about that.)