sd-bus: allow receiving messages with MSG_CTRUNC set

sd-bus: allow receiving messages with MSG_CTRUNC set

In the event that we can't receive all of the fds from the message
(which can happen for a number of reasons including LSM denials or
hitting the fd limit of the process) the kernel will set the MSG_CTRUNC
flag.  Through our use of recvmsg_safe() we've been treating this as a
fatal error, which will result in dropping the connection.

Let's dial that back a bit: we can receive the message, but when the
user attempts to access the missing fds via sd_bus_message_read_basic()
we can return the (existing) error code of -EBADMSG to indicate that the
fd is missing.

We can do this by using recvmsg() directly, and relaxing some of the
checks on message creation: when (and only when) we have received
MSG_CTRUNC we allow a smaller than expected (per the header) number of
fds to be present.  The error check in sd_bus_message_read_basic() was
already there so we don't need to do anything about that.

This puts the receiver of the message into a difficult situation: you
can call sd_bus_message_read_basic() as often as you want but as long as
it keeps returning -EBADMSG it won't progress through the message and
you won't be able to close whatever container you're in.  That means
that the user will probably need to abandon processing the message
anyway.  So why not just drop the message up front?  This approach is
more likely to yield a useful error message, which will be invaluable
for people trying to track down problems caused by LSM denials.

Fixes #34688