Add testing infrastruture bits for running with MI on a separate UI

With this, a specific test may can start GDB with MI on a separate UI
by using:

  mi_gdb_start separate-mi-tty

In addition, it's also possible to run the whole testsuite with MI on
a separate tty, with:

 make check RUNTESTFLAGS="FORCE_SEPARATE_MI_TTY=1"

gdb_main_spawn_id and mi_spawn_id are added so that tests may expect
output from either channel.

While at it, inferior_spawn_id was not being cleared when gdb exits,
unlike the other spawn ids, thus a test that starts gdb more than once
would end up using a stale spawn id.

gdb/testsuite/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* README (Testsuite Parameters): Document FORCE_SEPARATE_MI_TTY.
	* lib/gdb.exp (default_gdb_exit): Clear inferior_spawn_id.
	* lib/mi-support.exp (mi_uncatched_gdb_exit): Unset
	gdb_main_spawn_id, mi_spawn_id, unset inferior_spawn_id.
	(gdb_main_spawn_id, mi_spawn_id): Declare and
	comment.
	(mi_create_inferior_pty): New procedure,
	factored out from default_mi_gdb_start.
	(switch_gdb_spawn_id, mi_gdb_start_separate_mi_tty): New
	procedures.
	(default_mi_gdb_start): Call mi_gdb_start_separate_mi_tty if the
	separate-mi-tty option is specified, or SEPARATE_MI_TTY is set.
	Use mi_create_inferior_pty.
	(mi_gdb_start): Use eval to pass down args list.

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