Make tmpfs_snapshot.h usable in userland.

ok pedro@
Add tmpfsrd format stub.
some more wip
Make list/read work.

XXX: hardlinks are handled incorrectly
XXX: file names should contain the full path
Reconstruct the files full path.
snprintf() nit
malloc() -> calloc()
sort includes
add comment (hopefully temporary)
error messages
turn on some warnings for now
silly tmpfsrd_wr()
write header
Add read support for sockets & fifos.
Handle hardlinks in read path.
wip: write support

Not much yet, but produces a correct archive header and fills some
parts of the file headers.
Set type.
Use static splay macros.
Add missing TMPFS_SNAP_HTON().
Don't bother writing a header; just skip instead.