CHROMIUM: security: Yama: add link restrictions

CHROMIUM: security: Yama: add link restrictions

Add symlink and hardlink restrictions that have shown real-world security
benefits, along with sysctl knobs to control them.

BUG=chromium-os:22137
TEST=x86-alex build, boot, suite_Smoke passes, logging_UserCrash passes,
 security_SymlinkRestrictions, security_HardlinkRestrictions,
 security_ptraceRestriction.

Change-Id: I983e711f2f7c74b2f30d632b9fea4761637523e9
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/12407
Reviewed-by: Olof Johansson <olofj@chromium.org>
[ 3.4-rc5: added many #include to fix missing types --grundler]