Add the "n-flag-clear" option to the "segment-routing prefix"
command. The only thing that option does is to clear the node
flag of the Prefix-SID, even if it corresponds to a local loopback
address. No changes are necessary other than that in order to fully
support Anycast-SIDs. isisd already supports multiple routers
advertising the same route with the same Prefix-SID after the recent
refactoring. Clearing the node flag for such anycast routes isn't
strictly required, but failure to do so can lead to problems like
TI-LFA picking the wrong Prefix-SID when calculating repair paths.