[c-nsp] PIM dense mode fallback prevention on Catalyst

[christian.macnevin at uk.bnpparibas.com]

Hi,
Looking at dense mode fallback commands for PIM-SM, and it appears the 
command "no ip pim dm-fallback" has been
introduced in 12.3T.  What's the support like for the cats? Can't find a 
decent support matrix for it, but it seems like a no-brainer
for inclusion.
Cheers
Christian.