Add a new $_inferior convenience variable

Add a new $_inferior convenience variable

Like $_thread, but holds the current inferior number.

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

	* NEWS: Mention $_inferior.
	* inferior.c (inferior_id_make_value): New.
	(inferior_funcs): New.
	(_initialize_inferior): Create $_inferior variable.

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

	* gdb.base/default.exp: Expect $_inferior as well.
	* gdb.multi/base.exp: Test $_inferior.

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

	* gdb.texinfo (Inferiors and Programs): Document the $_inferior
	convenience variable.
	(Convenience Vars): Likewise.