nextcloud by default uses the `/config/` directory in the source/application tree for its config file(s).
with this commit that directory can be overridden by the `NEXTCLOUD_CONFIG_DIR` environment variable.

in uwsgi, you would use the option `--env "NEXTCLOUD_CONFIG_DIR=/tmp/nx-config/"`
in apache `SetENV …`
and the cli command can be run with: `NEXTCLOUD_CONFIG_DIR=/tmp/nx-config ./occ` (or just use `export` once in the
shell).

NEXTCLOUD_CONFIG_DIR can be supplied with or without the trailing slash (`/`), but in all cases `$configDir` will have
it automatically added if needed.

The other changes are several occurrences of `OC::$SERVERROOT . '/config'` to `OC::$configDir`.

* the setup check is still there

* makes debugging easier

that I noticed while looking at some code.

* method code is now in the static public namespace (5 sloc)

This reverts commit 6a1a4880.

use flock instead of just checking if there is a file to prevent deadlocks when the process crashed or the unlink wasnt done

move cron.lock into top directory to prevent external storage issues

use tmp directory lock file

use new config interface

incorporate #13770

add dots

maintain deprecated \OC::$session when getting or setting the session via the server container or UserSession

restore order os OC::$session and OC::$CLI

remove unneded initialization of dummy session

write back session when $useCustomSession is true

log warning when deprecated app is used

[error] 4461#0: *186285 FastCGI sent in stderr: "PHP message
: PHP Fatal error:  Call to a member function execute() on null in /var/www/ownc
loud/cron.php on line 125" while reading response header from upstream, client:
217.13.183.252, server: cloud.mycloud.com, request: "GET /cron.php HTTP/1.1", upstre
am: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "cloud.mycloud.com", referrer: "
https://cloud.mycloud.com/"

Fixes #2012