Fix #10926

handleRequest() is not called from remote.php or public.php which made these files party available but all included apps in there produced errors.

As the expected behaviour is anyways that a trusted domain warning is shown I moved this to init()

Fixes https://github.com/owncloud/core/issues/10064

If client does not send content length header on webdav upload (e.g. because the content comes from a stream and its length is not predictable) the put() method should not try to compare the content length value with the actually amount of received data, because this will always fail and results in a BadRequest exception. So the check will only be performed if $_SERVER['CONTENT_LENGTH'] is set.

Some providers block connections with missing user-agents. Also
user-agents are useful for analyzing requests. I've added a USER_AGENT
constant that is used in cURL and in file_get_contents.

OC_Util::generateRandomBytes() only returns lowercase alphanumeric values.
We should use the new RNG which has a broader characterset.

Fix issue where the currently logged user was causing side-effects when
upgrading.
Now setting incognito mode (no user) on update to make sure the whole
apps list is taken into account with getEnabledApps() or isEnabled().

header "User-Agent" was missed