* src/debug.c, src/debug.h: add keyboard debugging class.
* src/dumpkeys.c: refresh on Ctrl-L, reset on 'r'.
* src/keymap.c, src/keymap.h: add more setting combinations, and a simplistic
	check for more complete coverage.
* src/vte.c: add debug messages for keyboard handling.  Improve detection of
	invalid coordinates as a signal that no text is selected or hilited.
* termcaps/xterm.baseline: add a baseline copy of xterm from my termcap, so
	that I can diff it more easily.
* termcaps/xterm: remove key definitions -- xterm's way more complicated than
	the capability strings suggest.