sysctl: warn when separators are repeated

sysctl: warn when separators are repeated

For some reason sysctl earlier allowed quite strange separators.

% sysctl kernel./.pty.nr
kernel./.pty.nr = 6
% sysctl kernel///pty//////////nr
kernel...pty..........nr = 6

This commit does not disallow that sort of constructs, but will warn
about them. In future disallowing these might be reasonable thing to
do.

% sysctl kernel./.pty.nr
sysctl: separators should not be repeated: ./.pty.nr
kernel./.pty.nr = 6

Signed-off-by: Sami Kerola <kerolasa@iki.fi>