MINOR: fd: add functions to set O_NONBLOCK and FD_CLOEXEC

MINOR: fd: add functions to set O_NONBLOCK and FD_CLOEXEC

Instead of seeing each location manipulate the fcntl() themselves and
often forget to check previous flags, let's centralize the functions to
do this. It also allows to drop fcntl.h from most call places and will
ease the adoption of different OS-specific mechanisms if needed. Note
that the fd_set_nonblock() function purposely doesn't check the previous
flags as it's meant to be used on new FDs only.