New logstash-forwarder build

Back to the official git repo.
Preserve TERM env var in docker-enter
Update logstash-forwarder init script

Lots of changes effective in 0632ce3, some of which broke the init
script.

* read all files in a config dir
* -from-beginning is now the default (and unsupported)
* gratuitous: dynamic GOMAXPROCS
* use default spool size
* set cwd to /var/lib/logstash-forwarder so state file can always be
  found