CLEANUP: tree-wide: use fd_set_nonblock() and fd_set_cloexec()

CLEANUP: tree-wide: use fd_set_nonblock() and fd_set_cloexec()

This gets rid of most open-coded fcntl() calls, some of which were passed
through DISGUISE() to avoid a useless test. The FD_CLOEXEC was most often
set without preserving previous flags, which could become a problem once
new flags are created. Now this will not happen anymore.