librados: simplify/fix rados_pool_list bounds checks

We were not breaking out of the loop when we filled up the buffer unless
we happened to do so on a pool name boundary.  This means that len would
roll over (it was unsigned).  In my case, I was not able to reproduce
anything particularly bad since (I think) the strncpy was interpreting the
large unsigned value as signed, but in any case this fixes it, simplifies
the arithmetic, and adds a simple test.

- use a single 'rl' value for the amount of buffer space we want to
  consume
- use this to check that there is room and also as the strncat length
- rely on the initial memset to ensure that the trailing 0 is in place.

Fixes: #8447
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 3ec32a6bb11d92e36a0e6381b40ce2fd1fbb016a)