Add support for keyword commands.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>

Use the array_size() helper macro.  Replaces several instances of local
macros with the same definition.

Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

The correct method to link to NetSNMP is to use net-snmp-config (which
is like pkg-config). Explicit link to libcrypto is also dropped
(NetSNMP libs are linked to libcrypto, no need to link Quagga to
it). Moreover, @SNMP_INCLUDES@ is dropped because useless. Due to a
bug in configure.ac, it was properly populated.

Some .h files in lib/ are autogenerated. The search path should
include the build directory and the source directory. They usually
match but sometimes, they may be different. For example:

 $ mkdir build
 $ cd build
 $ ../configure
 $ make

The previous commits modified both zebra and bgpd for additional
SAFI field, but not any other routing daemon, which led to zebra
daemon crashing with failed assertion.

All daemons modified to support custom path to zserv
socket.

lib: generalize a zclient connection

zclient_socket_connect added. zclient_socket and
zclient_socket_un were hidden under static expression.
"zclient_serv_path_set" modified.

and use VTY_GET_INTEGER_RANGE instead of calling strtoul

IPv6 supports the same concept of differentiated service for routing
protocols as IPv4, but like too many things, the standards committee
decided that having two names for the same thing wasn't good enough and
introduced a third more generic term transport class.

The socket option to set transport class works the same as IPv4, but the
arguments are different.

* lib/sockopt.[ch]
  * setsockopt_ipv6_tclass(): new function
* bgpd/bgp_network.c
  * bgp_connect(): set socket option
  * bgp_listener(): set socket option
* ospf6d/ospf6_network.c
  * ospf6_set_transport_class(): new function
  * ospf6_serv_sock(): set socket option
* ripngd/ripngd.c
  * ripng_make_socket(): set socket option