Clean up the port

• Set LICENSE_FILE
• Switch to USES=localbase
• Simplify handling of SUB_FILES for option DRI3
• Fix a couple of typos in pkg-message

We should probably install /etc/X11/xrdp/xorg.conf as a @sample, but that's
probably something that the maintainer should take a look at.

Approved by: portmgr (blanket)