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