strv: when growing strv arrays piecemeal actually allocate memory in exponential...

strv: when growing strv arrays piecemeal actually allocate memory in exponential steps

Let's improve memory allocation for call such as strv_extend() that just
one item to an strv: these are often called in a loop, where they used
to be very ineffecient, since we'd allocate byte-exact space. With this
change let's improve on that, by allocating exponentially by rounding up
to the next exponent of 2. This way we get GREEDY_REALLOC()-like
behaviour without passing around state.

In fact this should be good enough so that we could replace existing
loops around GREEDY_REALLOC() for strv build-up with plain strv_extend()
and get similar behaviour.