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>

This change removes the previous OpenSSL/NSS hack in PKCS#12 parsing and
limits the hacks purely to the BER->DER conversion function, where they
belong.

PKCS#7 and #12 switch between implicit and explicit tags in different
places and sometimes only implicitly define that they are using implicit
tags. This change fixes a previous confusion where an implicit tag was
thought to be explicit.

Change-Id: Ib68c78cf2a1bfcbf90a296cb98313ab86ed2a1f3
Reviewed-on: https://boringssl-review.googlesource.com/1640
Reviewed-by: Adam Langley <agl@google.com>

Previously, the ASN.1 functions in bytestring were capable of processing
indefinite length elements when the _ber functions were used. That works
well enough for PKCS#3, but NSS goes a bit crazy with BER encoding and
PKCS#12. Rather than complicate the core bytestring functions further,
the BER support is removed from them and moved to a separate function
that converts from BER to DER (if needed).

Change-Id: I2212b28e99bab9fab8c61f80d2012d3e5a3cc2f0
Reviewed-on: https://boringssl-review.googlesource.com/1591
Reviewed-by: Adam Langley <agl@google.com>