gdb/mi: preserve user selected thread and frame when invoking MI commands

gdb/mi: preserve user selected thread and frame when invoking MI commands

Fix for PR gdb/20684.  When invoking MI commands with --thread and/or
--frame, the user selected thread and frame was not preserved:

  (gdb)
  info thread
  &"info thread\n"
  ~"  Id   Target Id                                           Frame \n"
  ~"* 1    Thread 0x7ffff7c30740 (LWP 19302) \"user-selected-c\" main () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:60\n"
  ~"  2    Thread 0x7ffff7c2f700 (LWP 19306) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ~"  3    Thread 0x7ffff742e700 (LWP 19307) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ^done
  (gdb)
  info frame
  &"info frame\n"
  ~"Stack level 0, frame at 0x7fffffffdf90:\n"
  ~" rip = 0x555555555207 in main (/home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:60); saved rip = 0x7ffff7c5709b\n"
  ~" source language c.\n"
  ~" Arglist at 0x7fffffffdf80, args: \n"
  ~" Locals at 0x7fffffffdf80, Previous frame's sp is 0x7fffffffdf90\n"
  ~" Saved registers:\n "
  ~" rbp at 0x7fffffffdf80, rip at 0x7fffffffdf88\n"
  ^done
  (gdb)
  -stack-info-depth --thread 3
  ^done,depth="4"
  (gdb)
  info thread
  &"info thread\n"
  ~"  Id   Target Id                                           Frame \n"
  ~"  1    Thread 0x7ffff7c30740 (LWP 19302) \"user-selected-c\" main () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:60\n"
  ~"  2    Thread 0x7ffff7c2f700 (LWP 19306) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ~"* 3    Thread 0x7ffff742e700 (LWP 19307) \"user-selected-c\" child_sub_function () at /home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30\n"
  ^done
  (gdb)
  info frame
  &"info frame\n"
  ~"Stack level 0, frame at 0x7ffff742dee0:\n"
  ~" rip = 0x555555555169 in child_sub_function (/home/uuu/gdb/gdb/testsuite/gdb.mi/user-selected-context-sync.c:30); saved rip = 0x555555555188\n"
  ~" called by frame at 0x7ffff742df00\n"
  ~" source language c.\n"
  ~" Arglist at 0x7ffff742ded0, args: \n"
  ~" Locals at 0x7ffff742ded0, Previous frame's sp is 0x7ffff742dee0\n"
  ~" Saved registers:\n "
  ~" rbp at 0x7ffff742ded0, rip at 0x7ffff742ded8\n"
  ^done
  (gdb)

This caused problems for frontends that provide access to CLI because UI
may silently change the context for CLI commands (as demonstrated above).

This commit fixes the problem by restoring thread and frame in
mi_cmd_execute (). With this change, there are only two GDB/MI commands
that can change user selected context: -thread-select and -stack-select-frame.
This allows us to remove all and rather complicated logic of notifying
about user selected context change from mi_execute_command (), leaving it
to these two commands themselves to notify.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=20684