Lots of small changes

Hello Jay,
while working on the Univention package I had a look at the files and tried to understand how it's supposed to work. While doing that I noticed several things, which I change:
- Some small build fixes and changes, which broke "make distcheck"
- A potential format string vulnerability
- simplification of the X11rdp build.xsh script
- missing manual pages for xrdp tools
Please have a close look and take the pices you like, the other ones I'm willing to rework.

One note about the manual pages: I tried to understand how the pices of xrdp work together and missed some high level overview, so I hope that I got it correct.

For the Univention package I'll have to change several things, so getting some options to configure them would be nice for a future version. For now I'll hack the C code, but if I find some more free time, I'll have a look at doing it properly:
- Replacing the bitmaps via .ini file.
- Changing the Texts (localization)
- Forcing a session type but keeping the login dialog for user name and password
- Hiding the session type combo box when only one type is available