MEDIUM: fd: prepare FD_POLL_* to move to bits 8-15

MEDIUM: fd: prepare FD_POLL_* to move to bits 8-15

In preparation of merging FD_POLL* and FD_EV*, this only changes the
value of FD_POLL_* to use bits 8-15 (the second byte). The size of the
field has been temporarily extended to 32 bits already, as well as
the temporary variables that carry the new composite value inside
fd_update_events(). The resulting fdtab entry becomes temporarily
unaligned. All places making access to .ev or FD_POLL_* were carefully
inspected to make sure they were safe regarding this change. Only one
temporary update was needed for the "show fd" code. The code was only
slightly inflated at this step.