fs: re-enable watch facility in AIX

On AIX, watch feature depends on AHAFS based Event infrastructure.
While in principle the watch use case is same across platforms, there
are subtle differences in the way AIX deals with this, with few
behavioral changes (external).

This commit addresses an assertion failure on folder watch, enabling the
AIX code for watch feature which was masked under a macro, open up
relevant test cases, skip tests which comes under the AIX limitation,
and make the document changes as appropriate.

Refs: https://github.com/nodejs/node/pull/11094
Refs: https://github.com/nodejs/node/issues/5085
PR-URL: https://github.com/nodejs/node/pull/10085


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>