Exit code of exited inferiors in -list-thread-groups

Exit code of exited inferiors in -list-thread-groups

Don't reset the exit code at inferior exit and print it in
-list-thread-groups.

gdb/ChangeLog:

	* NEWS: Announce new exit-code field in -list-thread-groups
	output.
	* inferior.c (exit_inferior_1): Don't clear exit code.
	(inferior_appeared): Clear exit code.
	* mi/mi-main.c (print_one_inferior): Add printing of the exit
	code.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-exit-code.exp: New file.
	* gdb.mi/mi-exit-code.c: New file.

gdb/doc/ChangeLog:

	* gdb.texinfo (Miscellaneous gdb/mi Commands): Document new
	exit-code field in -list-thread-groups output.