Since the Storage interface isn't ready to work directly with exceptions
like Forbidden and NotFound, the DAV storage has been adapted to still
return false when expected.

Can happen when trying to instantiate external storages that have
incomplete config, where the constructor throws an exception (the
exception is caught in createStorage())

Whenever an exception occurs during scan of a remote share, the share is
checked for availability. If the storage is gone, it will be removed
automatically.

Also, getDirectoryContent() will now skip unavailable storages.

It's better to encode the string to prevent possible (yet unknown) bugs in combination with PHP's type juggling.

Previously the boolean statements evaluated to either an empty string (false) or a not empty one (true, then it was 1). Not it always evaluates to false or true.

This also removes a stray - that was not intended there but shouldn't have produced any bugs. Just to increase readability.

Thanks @nickvergessen for spotting.

Addresses https://github.com/owncloud/core/pull/13235/files#r22852319

The internal path was matched without the last "/" which caused
"files_trashbin" to also match when the internal path was "files".

This adds the missing slash for the comparison.

Isset is a native language construct and thus A LOT faster than using strlen()

On my local machine this leads to a 1s performance gain for about 1 million paths. Considering that this function will be called a lot for every file operation this makes a noticable difference.

`normalizePath` is a rather expensive operation and called multiple times for a single path for every file related operation.

In my development installation with about 9GB of data and 60k files this leads to a performance boost of 24% - in seconds that are 1.86s (!) - for simple searches. With more files the impact will be even more noticeable. Obviously this affects every operation that has in any regard something to do with using OC\Files\Filesystem.

Part of https://github.com/owncloud/core/issues/13221

The check for invalid paths is actually over-complicated and performed twice resulting in a performance penalty. Additionally, I decided to add unit-tests to that function.

Part of https://github.com/owncloud/core/issues/13221

I was getting a lot of these in my logs for no apparent reason, and file
uploads were failing:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: ","level":4,"time":"2015-01-06T15:33:39+00:00"}

In order to debug it, I had to add unique messages to all the places where
this exception was thrown, to identify which one it was, and that made the
logs much more useful:

	{"app":"webdav","message":"Sabre\\DAV\\Exception\\ServiceUnavailable: Encryption is disabled","level":4,"time":"2015-01-06T15:36:47+00:00"}

fix comments and newline

add newline