Fix unreachable nc portability shim
This fixes nc failing to run on darwin due to it incorrectly setting the
linux-specific SOCK_NONBLOCK flag on connect.
nc already had a portability shim in apps/nc/compat/sys/socket.h, which
kicks in if SOCK_NONBLOCK is undefined. But that header includes
include/compat/sys/socket.h, which also has a portability shim that
defines a default value for SOCK_NONBLOCK if it's undefined. Thus the
first portability shim was unreachable.
Fixes this by moving the NEED_SOCKET_FLAGS flag into the outer shim, and
having the inner shim activate if NEED_SOCKET_FLAGS is defined.
This closes https://github.com/libressl-portable/portable/issues/631
