radvd.8.man: remove not regarding sysctl

The man page notes that radvd will not start if forwarding is not
enabled.  This hasn't been true for a while, so it's time to remove
the note.
Merge pull request #37 from abrodkin/master

better check for sysctl() availability
test/check.c: fix --mode arg
Merge remote-tracking branch 'origin/master'
Revert "better check for sysctl() availability"

This reverts commit 2b09fdf64a5f4eb1db940c21fa65c016176a1f79.
fixed: better check for sysctl() availability - builds on platforms with sysctl

This commit fixes an build issue on machines which actually still have sysctl.
As of now this is x86{_64} with glibc for example.

As it was mentioned before some Linux ports are dropping sysctl support.
This is intentional move and in future versions of Linux kernel this system
call will eventually go away. As a good alternative one may and should use
procfs (/proc/sys).

Currently we chech if corresponding header "sys/sysctl.h" exists.
But the problem happens if libc in use still installs "sys/sysctl.h" header
while provides no implementation for it.

This is exactly the case with uClibc and probably other implementations of libc.
But if we check for existing "sysctl" symbol in libc we're golden at least in
compile-time because libc provides some stub at least.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>