Update exabgp to version 3.4.11.

Pkgsrc changes: adapt PLIST; the healtcheck script moved

Upstream changes:

This release is a bug fix "mostly" release, back porting fixes
performed in master.

* Change: install healthcheck in bin
* Feature: Allow single line flow route requested by: Pavel Odintsov
* Feature: show route (extensive) can take a neighbor as parameter
  requested by: jtkdpu
* Fix: Do shutdown when waiting for a new connection to a peer
* Fix: Bad ASN enconding when ASN4 is not negotiated reported by:
  Orangefish on github
* Fix: Shutdown when waiting for a new outgoing connection to establish
* Fix: JSON counter reported by:
* Fix: JSON flow printing for source and destination
* Fix: Do not always locate exabgp.env reported by: Florian Obser
* Fix: Correctly drop root privileges reported by: Florian Obser
* Fix: validation of flow routes
* Fix: Python differences between Unix version breaking process forking
* Fix: Allow = with flowspec singleton reported by: Pavel Odintsov
* Fix: selfcheck feature
* Fix: do not refuse to parse multiple MP attributes in an update
* Fix: possible bug with attribute information due to caching

(he)