s3: Plumb smb_filename through SMB_VFS_STAT and SMB_VFS_LSTAT

s3: Plumb smb_filename through SMB_VFS_STAT and SMB_VFS_LSTAT

This patch introduces two new temporary helper functions
vfs_stat_smb_fname and vfs_lstat_smb_fname.  They basically allowed me
to call the new smb_filename version of stat, while avoiding plumbing
it through callers that are still too inconvenient.  As the conversion
moves along, I will be able to remove callers of this, with the goal
being to remove all callers.

There was also a bug in create_synthetic_smb_fname_split (also a
temporary utility function) that caused it to incorrectly handle
filenames with ':'s in them when in posix mode.  This is now fixed.