MINOR: fd: move .initialized into fdtab[].state

MINOR: fd: move .initialized into fdtab[].state

No need to keep this flag apart any more, let's merge it into the global
state. The bit was not cleared in fd_insert() because the only user is
the function used to create and atomically send a log message to a pipe
FD, which never registers the fd. Here we clear it nevertheless for the
sake of clarity.

Note that with an extra cleaning pass we could have a bit number
here and simply use a BTS to test and set it.