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.
(This used to be commit 1af8bf34f1caa3e7ec312d8109c07d32a945a448)