lib: Add the ability to grab a nhg starting id

lib: Add the ability to grab a nhg starting id

Add new function zclient_get_nhg_start that will allow an
upper level protocol to get a starting point for it's own
nhg space.  Give each protocol a space of 50 million.
zebra will own the space from 0 - 199999999 because
of SYSTEM, KERNEL and CONNECT route types.

This is the start of some work that will allow upper
level protocols to install and maintain their own NHG's.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>