gdbserver: allow the --debug command line option to take a value

gdbserver: allow the --debug command line option to take a value

Currently, gdbserver has the following command line options related to
debugging output:

  --debug
  --remote-debug
  --event-loop-debug

This doesn't scale well.  If I want an extra debug component I need to
add another command line flag.

This commit changes --debug to take a list of components.

The currently supported components are: all, threads, remote, and
event-loop.  The 'threads' component represents the debug we currently
get from the --debug option.  And if --debug is used without a
component list then the threads component is assumed as the default.

Currently the threads component actually includes a lot of output that
is not really threads related.  In the future I'd like to split this
up into some new, separate components.  But that is not part of this
commit, or even this series.

The special component 'all' does what you'd expect: enables debug
output from all supported components.

The component list is parsed left to write, and you can prefix a
component with '-' to disable that component, so I can write:

  target> gdbserver --debug=all,-event-loop

to get debug for all components except the event-loop component.

I've removed the existing --remote-debug and --event-loop-debug
command line options, these are equivalent to --debug=remote and
--debug=event-loop respectively, or --debug=remote,event-loop to
enable both components.

In this commit I've only update the command line options, in the next
commit I'll update the monitor commands to support a similar
interface.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Tom Tromey <tom@tromey.com>