tmon: Check log file for common secuirty issues

The tmon logging system blindly opens its log file on a static path, making it
very easy for someone to redirect that log information to inappropriate places
or overwrite other users data.  Do some easy checking to make sure we're not
logging to a symlink or a file owned by another user.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>