Provide default target methods for record targets that are likely to be shared

Provide default target methods for record targets that are likely to be shared
between different record targets.

gdb/
	* record.h (record_disconnect): New.
	(record_detach): New.
	(record_mourn_inferior): New.
	(record_kill): New.
	* record-full.c (record_disconnect, record_detach,
	record_mourn_inferior, record_kill): Move to...
	* record.c: ...here.
	(DEBUG): New.
	(record_stop): New.
	(record_unpush): New.
	(cmd_record_stop): Call record_stop. Replace unpush_target
	call with record_unpush call.
	(record_disconnect, record_detach): Assert that the target
	is of record stratum. Call record_unpush, record_stop, and
	DEBUG.
	(record_mourn_inferior, record_kill): Assert that the target
	is of record stratum. Call record_unpush and DEBUG.