Fix stubs to make them consistent for PHP 8 and add a checkKey() method along with a bug fix

[rlerdorf]

These are marked as nullable in the arginfo files in the tree. Without this change regenerating the arginfo with the PHP 8 gen_stubs will flip them to non-nullable.

I actually meant for these to be in separate pull requests, but I got lazy. Pick and choose here.

In the second part I added a userspace checkKey() method so you can check keys without a memcache roundtrip.
But in doing so I noticed that the checks don't match. libmemcached does an isgraph() check on ascii keys while the php-memcached check was iscntrl().
This breaks on many strings as per the test I added.

[sodabrew]

Thanks, Rasmus! I'll review soon.

[krakjoe]

Merged, thanks.

[sodabrew]

Thanks @krakjoe!

[SpencerMalone]

The iscntrl -> !isgraph is a pretty major change, memcache protocol does support non-ascii keys as outlined in https://github.com/memcached/memcached/blob/master/doc/protocol.txt by implicit statements such as:

The value of keys (and potentially other things) are "URI encoded". Since most
keys used conform to standard ASCII, this should have no effect. For keys with
less standard or binary characters, "%NN"'s are inserted to represent the
byte, ie: "n%2Cfoo" for "n,foo".

and the general key protocol definition:

Keys

---

Data stored by memcached is identified with the help of a key. A key
is a text string which should uniquely identify the data for clients
that are interested in storing and retrieving it. Currently the
length limit of a key is set at 250 characters (of course, normally
clients wouldn't need to use such long keys); the key must not include
control characters or whitespace.

[rlerdorf]

Commented in #546