[j-nsp] helper-only mode for graceful-restart

[Pekka Savola]

Hi,

AFAICS, JunOS allows either:
  1) no graceful-restart, no GR helper mode (default)
  2) enabling graceful-restart (and helper mode)

What JunOS does _not_ (AFAICS) support is enabling just the helper 
mode, but not graceful restart.

This would be extremely useful in the single-RE adjacent routers which 
should never do graceful restart themselves (because that could result 
in blackholing traffic), but could still assist the dual-RE routers in 
their graceful restart.

Have I understood the current features correctly?  Is this a feature 
others would like to see as well?