Add Python InferiorThread.global_num attribute

Add Python InferiorThread.global_num attribute

This commit adds a new Python InferiorThread.global_num attribute.
This can be used to pass the correct thread ID to Breakpoint.thread,
which takes a global thread ID, not a per-inferior thread number.

gdb/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* NEWS: Mention InferiorThread.global_num.
	* python/py-infthread.c (thpy_get_global_num): New function.
	(thread_object_getset): Register "global_num".

gdb/testsuite/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* gdb.multi/tids.exp: Test InferiorThread.global_num and
	Breakpoint.thread.
	* gdb.python/py-infthread.exp: Test InferiorThread.global_num.

gdb/doc/ChangeLog:
2016-01-13  Pedro Alves  <palves@redhat.com>

	* python.texi (Breakpoints In Python) <Breakpoint.thread>: Add
	anchor.
	(Threads In Python): Document new InferiorThread.global_num
	attribute.