Remove inline keyword

The compiler is smarter at deciding whether this is a good idea than we are...
Use the init_client() function to allocate a client.

This should really be moved to nbdsrv.c, but that's for later.
Protect this header against double inclusion
Register the first callback at startup
Mark the want_write variable when necessary
Don't recurse, makes no sense
Don't forget to initialize the "cur_file" descriptor