gdb, btrace: make record-btrace per-inferior

gdb, btrace: make record-btrace per-inferior

When there is more than one inferior, the "record btrace" command should
only apply to the current inferior.

gdb/ChangeLog:

2020-03-19  Markus Metzger  <markus.t.metzger@intel.com>

	* record-btrace.c (record_btrace_enable_warn): Ignore thread if
	its inferior is not recorded by us.
	(record_btrace_target_open): Replace call to all_non_exited_threads ()
	with call to current_inferior ()->non_exited_threads ().
	(record_btrace_target::stop_recording): Likewise.
	(record_btrace_target::close): Likewise.
	(record_btrace_target::wait): Likewise.
	(record_btrace_target::record_stop_replaying): Likewise.

gdb/testsuite/ChangeLog:

2020-03-19  Markus Metzger  <markus.t.metzger@intel.com>

	* gdb.btrace/multi-inferior.c: New test.
	* gdb.btrace/multi-inferior.exp: New file.