Support setting SELinux security contexts on modified files

On RedHat and related platforms, the Postfix package comes SELinux-enabled. The changes that this cookbook makes to Postfix config files cause them to lose their security context (postfix_etc_t) making them unreadable by the Postfix daemon.

This patch adds a boolean attribute (selinux) which, if set, causes the cookbook to attempt to set the security context for modified files. The attribute is true by default on RedHat, CentOS, and Fedora.