Add ta_fspec to struct tmpfs_args.

Let mount_tmpfs set ta_fspec; copy in to tmpfs_mount_t.
Add MNT_SNAPSHOT flag.

Allow it to be passed to VFS_MOUNT and clear afterwards. Adapt mount and
mount_tmpfs.
first attempt at intercepting read of /

completely untested.  if it compiles it's a bonus :)
wip: define img structs
wip: dump fs header
adjust offset & stop dumping when resid reaches zero.
Stop tapping tmpfs_read.
Some more snapshot bits and pieces.
wip: snapshot
progress so far for today.
Add filename to node dump
Dump content for VBLK, VCHR, VLNK and VREG nodes.