fix some syscall tests for 16-bit compat testing

Some syscall testcases have been compiled for 16-bit compat testing,
but doesn't change the code yet. This fix try to add proper code to let
them support this testing.

Signed-off-by: Han Pingtian <hanpt@linux.vnet.ibm.com>
Reviewed-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
setresuid/setresgid fixes

Previous commit "fix some syscall tests for 16-bit compat testing" introduced
16-bit support for setresuid/setresgid syscalls.

This commit:
1. Adds declarations of setresuid() and setresgid() functions to compat_16.h.
   It's to avoid compilation warnings.
   These functions are declared in the system's <unistd.h> if _GNU_SOURCE is
   defined. We can't simply add '#define _GNU_SOURCE' to compat_16.h, because
   <unistd.h> is included in multiple places, and compat_16.h is not the first.
   This solution is relatively easy and brings not much harm (IMHO).

2. setresuid02, setresuid03, setresuid04 do testing inside children, so it's
   appropriate to use tst_record_childstatus() there.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>