Patch tested by compiling with `__attribute__ ((format (printf, 2, 3)))`
applied to `cmd_results_new`.
String usage constants have been converted from pointers to arrays when
encountered. General handler format strings were sometimes modified to
include the old input string, especially for unknown command errors.
Implements rendering of borders. Title text is still to do.
Implements the following configuration directives:
* client.focused
* client.focused_inactive
* client.unfocused
* client.urgent
* border
* default_border
