Bruce Dawson pointed out that the shadowing of |ret| in |s3_srvr.c|
looked dodgy. It was actually deliberate (we don't want to reset the
default value of the function's |ret| variable with a successful return
from the callback) but it does look dodgy.

This change adds -Wshadow to ban variable shadowing and fixes all
current instances.

Change-Id: I1268f88b9f26245c7d16d6ead5bb9014ea471c01
Reviewed-on: https://boringssl-review.googlesource.com/2520
Reviewed-by: Adam Langley <agl@google.com>