pam configuration file for redhat

This comes from my X11RDP-RH-Matic work.

We need to install another `/etc/pam.d/xrdp-sesman` file on Red Hat and its clones.  I don't know how should I modify the `Makefile.am` but following changes are needed in addition to my commit.

- check if distribution is redhat (ex. existence of `/etc/redhat-release`)
- generate makefile which installs `instfiles/pam.d/xrdp-sesman.redhat` instead of `instfiles/pam.d/xrdp-sesman`