CHROMIUMOS: Escape | and & characters when passing to the shell.

CHROMIUMOS: Escape | and & characters when passing to the shell.

Add functions for hooks to check validity of domain names and paths.
Ensure we set a valid hostname, DNS domain and NIS domain.
Document the need for input validation in dhcpcd-run-hooks(8).

Fixes CVE-2011-996.
Based on patches to dhcpcd-3 by Marius Tomaschewski <mt@suse.de>;.

BUG=chromium-os:14192
TEST=Compile and run only -- this code path does not fire in ChromeOS.
Our version has all hooks removed so only the escape changes above apply.

Change-Id: Ia8c7b3e4af6e13b13319ee7f5cc58f3ab8e02b13

R=sleffler@chromium.org,sumit@chromium.org

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=cd306c5

[Sam Leffler, 2011-04-17 01:24:40]

LGTM