rdesktop 1.7.1

Reference: https://github.com/Homebrew/homebrew/issues/27947

Current rdesktop 1.8 does not work properly on OSX, in particular
keyboard and mouse events are not being sent. See official issue:
http://sourceforge.net/p/rdesktop/bugs/376/

This commit reverts to 1.7.1 and fixes an issue with missing
symbols.