Bind ENQ to "return-terminal-status". Fix an off-by-one which incorrectly

* src/caps.c: Bind ENQ to "return-terminal-status".
* src/iso2022.c: Fix an off-by-one which incorrectly triggered an assertion.
* src/vte.c: Implement send-primary/secondary-device-attributes.  Bind
	shift+insert to "paste PRIMARY".  Guard against NULL window/icon title
	when telling the child app what they are.