Use PS_DELETE then PS_ADD instead of PS_MOD with pollset_ctl
On AIX, pollset_ctl() PS_MOD command code only adds events to
a file descriptor, and doesn't remove them. This was causing
events that were no longer wanted to trigger callbacks that
didn't expect them.

This change removes the use of PS_MOD (at least for now) in
faviour of deleting and re-adding the file descriptors, as per
 the pollset_ctl documentation.