mount-tool: override mode of --tmpfs mounts to (rwxrwxrwx & ~umask)

mount-tool: override mode of --tmpfs mounts to (rwxrwxrwx & ~umask)

The kernel default for tmpfs mounts is rwxrwxrwt, i.e. world-writable with the
sticky bit set. This makes sense for /tmp and /var/tmp, but less so for other
directories. Users will not use systemd-mount to mount /tmp and /var/tmp, so
the existing behaviour is surprising. The defaults are overriden to the
same defaults we use for "internal tmpfs mounts", i.e. no sticky bit, nosuid,
nodev, and limits of size=10%, nr_inodes=400k.

--tmpfs is new, so we can modify the behaviour without breaking compatibility.