mountfsd: optionally accept an access mode for the dir created by MakeDirectory()

mountfsd: optionally accept an access mode for the dir created by MakeDirectory()

So far we expected the client to access the access mode themselves, and
set a restrictive 0700 server-side.

However, in some scenarios it's handy if the inode is created
server-side already with the right mode, in particular when the client
doesn't do anything with the dir just yet, and just needs it to be there
with the right mode. If it has to adjust the mode client-side it might
otherwise need to fork something off, join the selected userns, adjust
the mode, and kill the child off again.