make getservby*_r return error code rather than -1 (and using errno)

[musl] / src / thread / __futex.c

#include "futex.h"
#include "syscall.h"

int __futex(volatile int *addr, int op, int val, void *ts)
{
        return syscall(SYS_futex, addr, op, val, ts);
}