Fix kdbx nuke

[KeyWeeUsr]

I was trying to reproduce the #266 issue with Google Drive + Gnome but I couldn't find any relevant seek() that would break it within the construct library nor any file from the gvfs behaved strangely. If anything, the issue might need a better traceback because self.build_stream(...) calls further and it's still Python, so it shouldn't stop somewhere in the middle of the call stack if it's able to produce an OSError.

While the #266 isn't strictly a PyKeePass issue (and it's not reproducible clearly), I think leaving a free hand for the package to break the DB isn't really useful either. On top of that, some special files on Linux are actually sockets which may also happen to be used, thus it also makes sense to allow output to a socket.

# "server" / socket dumping the content
nc -lU socket.kdbx > file.kdbx

# client
from pykeepass import create_database
db = create_database("hello.kdbx")  # or open an existing one
db.save("socket.kdbx")
# socket then receives the bytes and dumps them to the "file.kdbx"

Closes #266

[Evidlo]

Sorry I let this languish. I implemented safe saving using BytesIO, which also means that we don't seek on the write medium.