Revert "src: fix _XOPEN_SOURCE redefinition warning"

This reverts commit 885142a5edc2c803fa8b9d92b5d0771379237764.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
Revert "constants: export O_NONBLOCK"

This reverts commit 00890e43fb935c8bc5dc150f0f2c96bc465d8a4d.

Signed-off-by: Trevor Norris <trev.norris@gmail.com>
constants: add O_NONBLOCK constant

It appears that it is defined unconditionally on all supported unixes.

fix #7867 #7855

Signed-off-by: Trevor Norris <trev.norris@gmail.com>