s3: vfs: add user_vfs_evg to connection_struct

s3: vfs: add user_vfs_evg to connection_struct

This will be used to in order to pass down the
impersonation magic from the SMB layer through
the SMB_VFS layer.

This includes the following options:

smbd:force sync user path safe threadpool
smbd:force sync user chdir safe threadpool
smbd:force sync root path safe threadpool
smbd:force sync root chdir safe threadpool

They can be used in order to test the non linux code
path on linux, once we get code that makes full use
of the new infrastructure.

Pair-Programmed-With: Stefan Metzmacher <metze@samba.org>

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Ralph Boehme <slow@samba.org>