Fix setting watchpoints when current thread is running

Currently, when the current thread is running, you can print global
variables.  However, if you try to set a watchpoint on the same
globals, GDB errors out, complaining that the selected thread is
running.  Like so:

 (gdb) c&
 Continuing.
 (gdb) p global
 $1 = 1098377287
 (gdb) watch global
 Selected thread is running.

This patch makes setting the watchpoint work.  You'll now get:

 (gdb) c&
 Continuing.
 (gdb) [New Thread 0x7ffff7d6e640 (LWP 434993)]
 [New Thread 0x7ffff756d640 (LWP 434994)]
 p global
 $1 = 88168
 (gdb) watch global
 Hardware watchpoint 2: global
 (gdb) [Switching to Thread 0x7ffff7d6e640 (LWP 434993)]

 Thread 2 "function0" hit Hardware watchpoint 2: global

 Old value = 185420
 New value = 185423
 int_return () at threads.c:39
 39      }

The problem is that update_watchpoint calls get_selected_frame
unconditionally.  We can skip it if the watchpoint expression is only
watching globals.

This adds a testcase that exercises both a...