net: Support inclusion of <linux/socket.h> before <sys/socket.h>

The following user-space program fails to compile:

    #include <linux/socket.h>
    #include <sys/socket.h>
    int main() { return 0; }

The reason is that <linux/socket.h> tests __GLIBC__ to decide whether it
should define various structures and macros that are now defined for
user-space by <sys/socket.h>, but __GLIBC__ is not defined if no libc
headers have yet been included.

It seems safe to drop support for libc 5 now.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Bastian Blank <waldi@debian.org>
Signed-off-by: David S. Miller <davem@davemloft.net>