Print current thread after loading a core file

Print current thread after loading a core file

downstream Fedora request:
	Please make it easier to find the backtrace of the crashing thread
	https://bugzilla.redhat.com/show_bug.cgi?id=1024504

Currently after loading a core file GDB prints:

Core was generated by `./threadcrash1'.
Program terminated with signal SIGSEGV, Segmentation fault.
8       *(volatile int *)0=0;
(gdb) _

there is nowhere seen which of the threads had crashed.  In reality GDB always
numbers that thread as #1 and it is the current thread that time.  But after
dumping all the info into a file for later analysis it is no longer obvious.
'thread apply all bt' even puts the thread #1 to the _end_ of the output!!!

Should GDB always print after loading a core file what "thread" command would
print?
[Current thread is 1 (Thread 0x7fcbe28fe700 (LWP 15453))]

BTW I think it will print the thread even when loading single/non-threaded
core file when other inferior(s) exist.  But that currently crashes
	[Bug threads/12074] multi-inferior internal error
	https://sourceware.org/bugzilla/show_bug.cgi?id=12074
plus I think that would be a correct behavior anyway.

gdb/ChangeLog
2015-01-22  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* corelow.c (core_open): Call also thread_command.
	* gdbthread.h (thread_command): New prototype moved from ...
	* thread.c (thread_command): ... here.
	(thread_command): Make it global.