This is a version of PKCS7_get_certificates but does not require
crypto/x509.

BUG=54

Change-Id: I20152a8d1f3ed866d47e41fe576ea9f442490224
Reviewed-on: https://boringssl-review.googlesource.com/15129
Reviewed-by: David Benjamin <davidben@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

A follow-up change will add a CRYPTO_BUFFER variant. This makes the
naming match the header and doesn't require including x509.h. (Though
like ssl.h and pkcs8.h, some of the functions are implemented with code
that depends on crypto/x509.)

Change-Id: I5a7de209f4f775fe0027893f711326d89699ca1f
Reviewed-on: https://boringssl-review.googlesource.com/15128
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>