btrace: allow full memory and register access for non-replaying threads

btrace: allow full memory and register access for non-replaying threads

The record btrace target does not allow accessing memory and storing registers
while replaying.  For multi-threaded applications, this prevents those
accesses also for threads that are at the end of their execution history as
long as at least one thread is replaying.

Change this to only check if the selected thread is replaying.  This allows
threads that are at the end of their execution history to read and write
memory and to store registers.

Also change the error message to reflect this change.

gdb/
	* record-btrace.c (record_btrace_xfer_partial)
	(record_btrace_store_registers, record_btrace_prepare_to_store):
	Call record_btrace_is_replaying with inferior_ptid instead of
	minus_one_ptid.
	(record_btrace_store_registers): Change error message.