Create inferior for tfile target

Create inferior for tfile target

When a trace file is loaded in Eclipse, it is expected to see thread
and process (=thread-group-started and =thread-created).  Create an
inferior and add a thread for this purpose.

This patch just reverts my previous patch.

gdb/testsuite:

2014-02-05  Yao Qi  <yao@codesourcery.com>

	Revert this patch:

	2013-05-24  Yao Qi  <yao@codesourcery.com>

	* gdb.trace/tfile.exp: Test inferior and thread.

gdb:

2014-02-05  Yao Qi  <yao@codesourcery.com>

	Revert this patch:

	2013-05-24  Yao Qi  <yao@codesourcery.com>

	* tracepoint.c (TFILE_PID): Remove.
	(tfile_open): Don't add thread and inferior.
	(tfile_close): Don't set 'inferior_ptid'.  Don't call
	exit_inferior_silent.
	(tfile_thread_alive): Remove.
	(init_tfile_ops): Don't set field 'to_thread_alive' of
	tfile_ops.