There might be the case that `fopen($file, 'r')` returns false and thus ownCloud might believe that the config file is empty and thus potentially leading to an overwrite of the config file.

This changeset introduces `file_exists` again which was used in ownCloud 5 where no such problems where reported and should not be affected by such problems.

Ref https://github.com/owncloud/core/issues/12785#issuecomment-71548720

This reduces the number of file writes we do for config.php and therefor
hopefully helps lowering the chances for empty config.php files

The permissions are already catched properly on the installation so we just have to check whether the file is readable to prevent fatal errors from happening.

Fixes https://github.com/owncloud/core/issues/12135

Potentially fixes https://github.com/owncloud/core/issues/11678

This adds a file lock to the config in hope that this prevents race conditions as reported in https://github.com/owncloud/core/issues/11070

Testplan:

- [ ] Delete config.php and make it read-only => Error is thrown that it is not writeable
- [ ] Installation still works
- [ ] Changing config settings works (i.e. using the SMTP config switches in the administration menu)
- [ ] Your PC didn't blow up
- [ ] Installing the news app and the "Disable AppCode checker" app did not destroy your installation

Only skip the main config

Otherwise read only additional configs might not be processed

Test on tmpdir

* test case added to avoid adding them later

is shown.

This patch was automatically generated as part of the following inspection:
https://scrutinizer-ci.com/g/owncloud/core/inspections/cdfecc4e-a37e-4233-8025-f0d7252a8720

Enabled analysis tools:
 - PHP Analyzer
 - JSHint
 - PHP Copy/Paste Detector
 - PHP PDepend

Conflicts:
	lib/private/vcategories.php

This reverts commit 8f93490a.

Squashed commit of the following:

commit eed4b49cebcbcc252a75ed85097730b73213b0da
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Mon Jul 8 10:03:23 2013 +0200

    initialize OC_Defaults only once

commit bf6f07ccc8fb87535a069ca341789a590cb187ee
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Mon Jul 8 10:02:48 2013 +0200

    link to doc in error messages

commit ea61ee60e06ee98f2671aec1fdaff666c50f47c2
Merge: e41af3d 3c1308fb
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Thu Jul 4 11:10:56 2013 +0200

    Merge branch 'master' into better-messages

commit e41af3d7bad26aa5ca9ab21ec7dcbadd3cfe5d4f
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Thu Jul 4 11:10:41 2013 +0200

    move to non-static defaults.php

commit 9e4258b1905244bdf34943a825421f041cbed43d
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Tue Jul 2 10:32:13 2013 +0200

    no sprintf here, lets t() handle it

commit fe1df349e248667a137f70d78b04225e5b42a111
Merge: d8f6859 cb5811bc
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Mon Jul 1 11:13:38 2013 +0200

    Merge branch 'master' into better-messages

commit d8f68595df2dc0e0917d916cbde511ec5333010b
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Mon Jul 1 11:10:57 2013 +0200

    use document base url from defaults.php

commit 6c5403748a45717125a2aa375550f05646317d72
Merge: bea6b1c 7b0e3e63
Author: Björn Schießle <schiessle@owncloud.com>
Date:   Mon Jul 1 11:00:15 2013 +0200

    Merge branch 'master' into better-messages

commit bea6b1c8a0968846065b9153e0a3f46a4e3245ee
Author: Jan-Christoph Borchardt <hey@jancborchardt.net>
Date:   Mon Jun 24 16:21:12 2013 +0200

    link to docs in WebDAV message, ref #3791

commit 3119b364a1094769e711283d1ce9014505f64ab9
Author: Jan-Christoph Borchardt <hey@jancborchardt.net>
Date:   Mon Jun 24 16:10:21 2013 +0200

    link to docs in error messages, fix #3819

This reverts commit 969e43c8.