gdbsupport: allow passing format string to scoped_debug_start_end

gdbsupport: allow passing format string to scoped_debug_start_end

A little thing that bothers me with scoped_debug_start_end is that it's
not possible to pass a format string to add context to the messages: the
start and end messages are fixed.

It was done like this at the time because there's the risk that debug
output is not enabled on entry (when the constructor runs) but is
enabled on exit (when the destructor runs).  For example, a user
debugging from a top-gdb may manually enable a debug_foo variable.  If
debug output is disabled while the constructor runs, we won't render the
format string (to minimize overhead) so it won't be available in the
destructor.

I think it would be nice to be able to use a format string along with
scoped_debug_start_end, and I think it's unfortunate that such a narrow
use case prevents it.  So with this patch, I propose that we allow
passing a format string to scoped_debug_start_end, and if the rare
situation described above happens, then we just show a "sorry, message
not available" kind of message.

The following patch makes use of this.

gdbsupport/ChangeLog:

	* common-debug.h (struct scoped_debug_start_end)
	<scoped_debug_start_end>: Change start_msg/end_msg for
	start_prefix/end_prefix.  Add format string parameter and make
	variadic.
	<~scoped_debug_start_end>: Adjust.
	<m_end_msg>: Rename to...
	<m_end_prefix>: ... this.
	<m_with_format>: New.
	<m_msg>: New.
	(scoped_debug_start_end): Make variadic.
	(scoped_debug_enter_exit): Adjust.

Change-Id: I9427ce8877a246a46694b3a1fec3837dc6954d6e