replaced stdio in many parts of samba with a XFILE. XFILE is a cut-down

replaced stdio in many parts of samba with a XFILE. XFILE is a cut-down
replacemnt of stdio that doesn't suffer from the 8-bit filedescriptor
limit that we hit with nasty consequences on some systems

I would eventually prefer us to have a configure test to see if we need
to replace stdio, but for now this code needs to be tested widely so
I'm enabling it by default.