Implemented undefined keyboard functions in Xlib backend.

keyboard_set_map and related functions. Tested with this sample game:
https://drive.google.com/file/d/0B1P7NepPcOslbkZJQk5VUVdCdmc/edit?usp=sharing

keyboard_clear. Tested with this sample game:
https://drive.google.com/file/d/0B1P7NepPcOslanpWcE11eDBJWTQ/edit?usp=sharing

All implementations are the same as their Win32 equivalents. I removed usermap and replaced it with keybdmap, as usermap was currently only returning identity values. The shared code could easily be pulled out into the General shared code, but I am not sure what your plans are for Android, etc., so I kept it separate. 

Note the behavior of keyboard_clear according to this bug report; it is somewhat counter-intuitive, but my test case behaves the same in Enigma and GM Studio:
http://bugs.yoyogames.com/view.php?id=07055