[RFC 0/6] MI notification of command option change

[Yao Qi <yao@codesourcery.com>]

When Eclipse connects to GDB through MI, user is still abel to
type command in console (or telnet session), so the state of GDB
may be changed while Eclipse is not aware of.  GDB is lack of
notifications to MI front end and/or telnet session when the
internal state of GDB is changed (eg. changed by some 'set foo bar'
commands or tracepoint is downloaded).

This patch series is to address the problem that caused by
command 'set foo bar'.  When user types some interesting commands
in console, GDB will send MI notifications about the command
option changes.  The following patch series will address the
problem caused by other aspects in the future.  In this way,
Eclipse is aware of the change of GDB state, and refresh or
update itself accordingly.

Patch 1/6 - 3/6 are about the new observer 'command_option_changed'
and new MI notification '=option-changed' when observer is notified.
Patch 4/6 - 6/6 are about registering three types of command
respectively.  Three commands will send MI notifications if their
option is changed.  There are more than three commands that can
change the state of GDB, but these three are reprsentives of
them.  The rest of commands can be easily changed to sending
MI notifications.

Regression tested on x86_64 native/gdbserver.