Fix four latent bugs found by audit, and make the shared-secret check reachableg fixes

[ispielma]

Five independent bug fixes turned up while Claude audited this repository. Each is a
separate commit so any one can be dropped.

Four are unambiguous defects. The fifth (the last) is a real bug whose fix
changes behaviour for one configuration, and I have flagged it clearly so you
can decide.

1. zlock.py passes an int port to subprocess

get_config() falls back to zprocess.zlock.DEFAULT_PORT, an int, when the
labconfig has no [ports] zlock entry, and that value goes straight into an
argv list:

TypeError: expected str, bytes or os.PathLike object, not int

So labscript-zlock cannot start from a labconfig that omits the port.
zlog.py and remote.py already wrap their ports with str().

2. excepthook.set_logger() recurses if called twice

set_logger() stashed the currently-installed handler into
warnings._showwarning before installing logwarning. On a second call the
stash captures logwarning itself, so it recurses into itself and the next
warning raises RecursionError. Reachable whenever a process installs the
excepthook more than once. Capturing the real handler once at import is also
idempotent, and stops writing to a private-looking name in warnings.

3. properties._default() cannot serialise np.floating or np.bool_

_default() only handled np.integer. np.float64 survives by accident
because it subclasses Python float, which masked the gap:

int64      ok
float64    ok
float32    TypeError
bool_      TypeError

Any device or plugin writing an np.float32 or np.bool_ into shot
properties fails in serialise().

4. Context.socket() tests against SecureContext, not SecureSocket

The comment directly above says the intent is to insert the security kwargs
only when the socket will be a SecureSocket. The test used SecureContext,
a Context class that can never be a socket class:

issubclass(zmq.Socket,   SecureContext) = False    SecureSocket = False
issubclass(SecureSocket, SecureContext) = False    SecureSocket = True

The common pyzmq 25 path, where ThreadAuthenticator passes zmq.Socket,
gives the intended answer either way — which is why this went unnoticed. A
caller explicitly passing SecureSocket silently lost its allow_insecure
configuration.

5. Shared-secret guard is unreachable when allow_insecure is configured — please read

The guard raising _ERR_NO_SHARED_SECRET sits inside the except clause, so
it only runs when [security] allow_insecure is absent. A labconfig that
explicitly sets

[security]
allow_insecure = False

with no shared_secret skips it entirely. Inside the except the condition
was also dead weight, since allow_insecure had just been set to False on
the line above.

_ERR_NO_SHARED_SECRET states the intended contract — configure a
shared_secret, or opt out with allow_insecure = True — so explicitly
writing allow_insecure = False without a secret is exactly the
misconfiguration the message exists to catch.

This changes behaviour. An installation with allow_insecure = False and
no shared secret now fails at startup with that message instead of continuing.
Such a setup previously worked as long as all traffic stayed on loopback,
because zprocess only enforces this at send time
(zprocess/security.py). Those users would need to supply a shared secret or
set allow_insecure = True.

It is the last commit on the branch, so drop 635aea9 if you would rather not
take that trade-off; the other four are independent of it.