[xmlsec] core methods for write of <X509SubjectName/> and <X509IssuerSerial/>

[Aleksey Sanin]

> Might is not easy to implement writing of  <X509Data>.
> Only reasonable please to specify "cert. xmlsec_write_type" is might 
> method XXX-AdoptCert (cert_store, cert, cert_properties).

Nobody said it is simple :) I would have implemented it myself otherwise :)
Anyway, I think that we can go with "all certs are written out in the 
same way"
approach as I have described before. If someone needs a really tricky
output then s/he can do it manually. Nobody said that <X509Data/> node
could not be written by application, not xmlsec :)

Aleksey