*: remove --enable-tcp-zebra, rework ZAPI path

*: remove --enable-tcp-zebra, rework ZAPI path

This adds "@tcp" as new choice on the -z option present in zebra and the
protocol daemons.  The --enable-tcp-zebra option on configure is no
longer needed, both UNIX and TCP socket support is always available.

Note that @tcp should not be used by default (e.g. in an init script),
and --enable-tcp-zebra should never have been in any distro package
builds, because

**** TCP-ZEBRA IS A SECURITY PROBLEM ****

It allows arbitrary local users to mess with the routing table and
inject bogus data -- and also ZAPI is not designed to be robust against
attacks.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>