[xmlsec] Binary-Hex addition

[Aleksey Sanin]

Remy,

I have changed your functions a little bit to match xmlsec API style. And
when I did it I found a natural place for these functions: xmlSecBuffer 
"class" :) Thus there is no need to add binhex.* files. I also have replaced
the CryptBinaryToString() calls in mscrypto/msx509.c with calls to new
functions (and that eliminated some code there :) ). All changes are 
checked
in the mscrypto branch.

I would appreciateif you can take a look at my changes to the "hex"
functions to make sure that I did not break something by mistake.

Thanks for the patch!
Aleksey