pkgman: Add --debug option for most commands

* Add CommonOptions class and add an instance to Command. It supports a
debug level.
* full-sync, install, search, uninstall, update: Add option --debug.
This allows getting some debug output from the solver in cases where
the result seems weird.

pkgman: Organize commands by category

pkgman: C++-ify the command handling

There's now a Command class that must be derived and registered with
a CommandManager, all simplified by a REGISTER_COMMAND macro. That gets
rid of the print_command_usage_and_exit() function copy for every
command, moves the short usage texts to the command implementations,
and avoids any repetition of the command name. When implementing a new
command only a new source file needs to be created, nothing else needs
to be touched.

pkgman: Add --debug option for most commands

* Add CommonOptions class and add an instance to Command. It supports a
debug level.
* full-sync, install, search, uninstall, update: Add option --debug.
This allows getting some debug output from the solver in cases where
the result seems weird.

pkgman: Organize commands by category

pkgman: C++-ify the command handling

There's now a Command class that must be derived and registered with
a CommandManager, all simplified by a REGISTER_COMMAND macro. That gets
rid of the print_command_usage_and_exit() function copy for every
command, moves the short usage texts to the command implementations,
and avoids any repetition of the command name. When implementing a new
command only a new source file needs to be created, nothing else needs
to be touched.