core: implement runtime flag to trace sync io

Use the --trace-sync-io flag to print a stack trace whenever a sync
method is used after the first tick, excluding during the process exit
event. (e.g. fs.readFileSync()) It does not track if the warning has
occurred at a specific location in the past and so will print the
warning every time.

Reason for not printing during the first tick of the appication is so
all necessary resources can be required. Also by excluding synchronous
calls during exit is necessary in case any data needs to be logged out
by the application before it shuts down.

Fixes: https://github.com/nodejs/io.js/issues/1674
PR-URL: https://github.com/nodejs/io.js/pull/1707


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Petka Antonov <petka_antonov@hotmail.com>