Tools: add setresuid syscall

Tools: add setresuid syscall

See `man 2 setresuid`. The code was tested with small UIDs (109) and
one which does not fit into 16 bit (100000000), since I wasn't too
sure about the workings of setresuid vs. setresuid32 (see NOTES of
the manpage) - it worked with both.

Linux on amd64 has only a single setresuid, as it the 64 bit arch
came after the setresuid32 syscall, and thus it started with the
later one as single common one

Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>