MB-10086:[cbcollect_info] added hidden --watch-stdin flag

So that we can spawn this thing from erlang and make it exit if erlang
asks it to

Change-Id: I5e7b31c4a988ec964eed4598dbf8ae458cb2dfb0
Reviewed-on: http://review.couchbase.org/37820
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>