In order to minimise the upstream diffs needed for bits of Android to
build with BoringSSL, this change implements the old style PKCS#12
functions as wrappers around the modern parser.

The function to read all the contents of a BIO could almost be a utility
function but I'll wait until there are two uses for it first.

The important change from the original functions is that these will
always read the complete buffer/BIO/FILE passed in. Based on a survey of
uses of d2i_PKCS12 that I found, this appears to be universally what
callers want anyway.

Change-Id: I3f5b84e710b161d975f91f4d16c83d44371368d1
Reviewed-on: https://boringssl-review.googlesource.com/1791
Reviewed-by: Adam Langley <agl@google.com>