add pwritev() support in posix module
This call is used heavily by ompio (the ROMIO alternative for Open MPI) at least, which means that Darshan cannot observe posix level writes:
We should of course also catch preadv() and other similar variants as well.
I'm tagging this as 3.2.2 but it can slide to a later point release. I'm really using that milestone to mean "soon" rather than that specific point release if it is ready soon.