identification: Change abbreviation for surname/serialNumber RDNs

identification: Change abbreviation for surname/serialNumber RDNs

To align with RFC 4519, section 2.31/32, the abbreviation for surname
is changed to "SN" that was previously used for serialNumber, which does
not have an abbreviation.

This mapping had its origins in the X.509 patch for FreeS/WAN that was
started in 2000.  It was aligned with how OpenSSL did this in earlier
versions.  However, there it was changed already in March 2002 (commit
ffbe98b7630d604263cfb1118c67ca2617a8e222) to make it compatible with
RFC 2256 (predecessor of RFC 4519).

Co-authored-by: Tobias Brunner <tobias@strongswan.org>
Closes strongswan/strongswan#179.