Pull up following revision(s) (requested by maya in ticket #856):

sys/nfs/nfs.h: revision 1.76
sys/nfs/nfs_subs.c: revision 1.230
sys/nfs/nfs_socket.c: revision 1.199
sys/nfs/nfs_clntsocket.c: revision 1.6

PR/40491: From Tobias Ulmer in tech-kern@:
1. Protect the nfs request queue with its own mutex
2. make the nfs_receive queue check for signals so that intr mounts
  can be interrupted.

XXX: pullup-8

(martin)