opal: minor update to guess_strlen

This is a minor update to
open-mpi/ompi@c52601f0c563132284cd1444c7a7039ba0032872.

If we have vsnprintf(), we might as well not have the rest of the
guess_strlen() routine.  Also document the nifty trick/behavior of
vsnprintf() that enables this shortcut (it was new to me!).