Add nodefs.Mount - utility to mount root over mountpoint with given
options. We already had nodefs.MountRoot, but that was taking only
nodefs.Options and there was no way to pass fuse.MountOptions in. The
new utility accepts both fuse.MountOptions and nodefs.Options, each
covering their level.

This should be generally useful(*), as well as it will be used in a next
patch in TestFopenKeepCache where fuse.MountOptions.PreciseDataCacheControl
will need to be used.

(*) see e.g. https://lab.nexedi.com/kirr/wendelin.core/blob/8f497094/wcfs/misc.go#L258
as example that users unroll their Mount versions to be able to pass in
fuse.MountOptions.