iproute2: Auto-detect the presence of setns in libc

iproute2: Auto-detect the presence of setns in libc

If libc has setns present use that version instead of
rolling the syscall wrapper by hand.

Dan McGee found the following compile error:

    gcc -D_GNU_SOURCE -O2 -Wstrict-prototypes -Wall -I../include
    -DRESOLVE_HOSTNAMES -DLIBDIR=\"/usr/lib/\"   -c -o ipnetns.o ipnetns.c
    ipnetns.c:31:12: error: static declaration of ‘setns’ follows non-static
    declaration
    /usr/include/bits/sched.h:93:12: note: previous declaration of ‘setns’
    was here
    make[1]: *** [ipnetns.o] Error 1

Reported-by:  Dan McGee <dan@archlinux.org>
Tested-by:  Dan McGee <dan@archlinux.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>