add options to disable the check for a working .htaccess file in data and for a working WebDAV server. This are advanced settings that are needed in special situations where our check fail and the user runs into an http timeout.

Fixes #5357

correctly expire cache if version file changed. Fixes problem that ownCloud only starts upgrading during login

Conflicts:
	lib/private/vcategories.php

SabreDAV uses cURL for the requests and as this does not honour
default_socket_timeout, setting it is useless and confusing as
people will expect the request to timeout faster than it actually
will do.

One has to use
 curl_setopt($curl, CURLOPT_TIMEOUT, x)
or
 curl_setopt($curl, CURLOPT_CONNECTTIMEOUT, x)
but there is currently no way to make SabreDAV pass this to cURL.

needs to log-in again in order to decrypt his private key with his password

which reviewer did not test the pull request?

 - Use camelCase

 - Add some phpdoc

 - Fix some indents

 - Use some more spacing

The return value almost never changes but this function
is called for every request.

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

Conflicts:

	lib/mail.php