MEDIUM: fd: remove the EV_FD_COND_* primitives

MEDIUM: fd: remove the EV_FD_COND_* primitives

These primitives were initially introduced so that callers were able to
conditionally set/disable polling on a file descriptor and check in return
what the state was. It's been long since we last had an "if" on this, and
all pollers' functions were the same for cond_* and their systematic
counter parts, except that this required a check and a specific return
value that are not always necessary.

So let's simplify the FD API by removing this now unused distinction and
by making all specific functions return void.