Make r/w servers unmount themselves cleanly upon receiving SIGINT/TERM.

Remove threading ifdefs. It was never easy to run the nfs client
without threads, but now it's even more difficult due to sillyrename
using workqueue, so whoever wants to do it will have to take to
manual fiddling.

Use write through cache here too. Doesn't make the same kind of
difference as with smbfs since nfs is "stateless", but better to
have the data on the server ASAP in case some other nods wants to
use it.

Default to threading, non-threaded debug mode can be compiled separately.
Useful if running this from fstab.

Don't compile diagnostic code which accesses kernel symbols directly.
Will fail now since not all archs have the same symbol names.

For applications which naughtily reach over to the rump kernel
namespace without using proper interfaces, do a little namespace
compensation.

Do some nasty manual tricks and treats to avoid having to call
rump_init() here (creating threads before we ask p2k to detach from
console is bad).

Try to honor rump_threads by not creating iothreads if threads are off.

Don't set PUFFS_FLAG_OPDUMP by default.

Add & enable rump_nfs.