Fix decoding of sysctl() when oldval fields are NULL

Fix decoding of sysctl() when oldval fields are NULL

If you call glibc's syscall wrapper like so:
	static int name[] = { CTL_NET, NET_IPV4, NET_IPV4_LOCAL_PORT_RANGE };
	int buffer[2] = { 32768, 61000 };
	size_t size = sizeof(buffer);
	sysctl(name, 3, 0, 0, buffer, size);
(note that oldval/oldlenp are NULL).

The current strace code complains like so:
_sysctl({{CTL_NET, NET_IPV4, NET_IPV4_LOCAL_PORT_RANGE, 38}, 3, process_vm_readv: Bad address
(nil), 0, 0x7fffe23c3960, 8}) = -1 EACCES (Permission denied)

Since passing NULL for the old values is valid, handle that explicitly.
This also simplifies the code a bit by splitting up the handling of the
new and old args so that we only handle the new args once.

Now the output looks like:
_sysctl({{CTL_NET, NET_IPV4, NET_IPV4_LOCAL_PORT_RANGE, 38}, 3, NULL, 0, 0x7fff8c0c91b0, 8) = -1 EACCES (Permission denied)

* system.c (sys_sysctl): Check if info.oldval is NULL first.  Move the
processing of oldlen/info.newval/info.newlen out so they always get
executed.  Fix the format strings so we use %lu for unsigned long rather
than a mix of %ld and %lu.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>