[WIP] CSRP compability implementation

[feliwir]

These changes add (server) compability to the CSRP library in it's non-conforming RFC5054 mode (which was the original behaviour). See https://github.com/cocagne/csrp :

NOTE This SRP implementation was created before the hashing algoritim specified in RFC 5054 became the de-facto standard for interoperable SRP implementations. 
The rfc5054_compat branch of this repository uses the RFC 5054 hashing algorithms and is known to be compatible with other SRP implementations. 
If this version works for you, please consider submitting a patch to this library that implements both the original default and the RFC 5054 implementation to allow a single mainline version of this library going forward.