Fix PEM parsing on Linux when using X509Certificate::FORMAT_AUTO
When using X509Certificate::FORMAT_AUTO, parsing is attempted optimistically first by seeing if the data decodes as any of the supported binary formats. The NSS routine used to handle PKCS#7 data is actually a generic routine, capable of supporting multiple formats.
When a PEM encoded certificate is decoded using PKCS#7, as is the case with FORMAT_AUTO, NSS will, upon encountering a failure to parse as PKCS#7, attempt to PEM decode the data and parse as either a certificate or a PKCS#7 structure. The problem with NSS's implementation is that it only decodes a single certificate, rather than all of the certificates in the file, preventing a full PEM chain from being read in.
The solution is to no longer optimistically try to decode the data as binary prior to trying to decode as PEM, and instead unconditionally attempt decoding as PEM-wrapped data before falling back to binary. This allows our handling routines to properly parse all of the supported PEM types, before the underlying crypto library is exposed to the data.