pam_motd: add support for a motd.d directory (#48)

pam_motd: add support for a motd.d directory (#48)

Add a new feature to pam_motd to allow packages to install their own
message files in a "motd.d" directory, to be displayed after the primary
motd.

Add an option motd_d= to specify the location of this directory.

Modify the defaults, in the case where no options are given, to display
both /etc/motd and /etc/motd.d.

Fixes #47

 * modules/pam_motd/pam_motd.c: add support for motd.d
 * modules/pam_motd/pam_motd.8.xml: update the manpage