zebra: don't raise privileges when creating unix zserv socket

zebra: don't raise privileges when creating unix zserv socket

Raising privileges is only necessary when binding to a TCP/UDP privileged
port (< 1024).

This solves a problem where the zserv.api socket was being created with
root ownership, preventing the client daemons to connect to zebra.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>