gdb: use option framework for add-inferior and clone-inferior

gdb: use option framework for add-inferior and clone-inferior

Convert the add-inferior and clone-inferior commands to make use of
the option framework.  This improves the tab completion for these
commands.

Previously the add-inferior command used a trick to simulate
completion of -exec argument.  The command use filename completion for
everything on the command line, thus you could do:

  (gdb) add-inferior /path/to/some/fil<TAB>

and GDB would complete the file name, even though add-inferior doesn't
really take a filename as an argument.  This helped a little though
because, if the user did this:

  (gdb) add-inferior -exec /path/to/some/fil<TAB>

then the file name would be completed.  However, GDB didn't really
understand the options, so couldn't offer completion of the options
themselves.

After this commit, the add-inferior command makes use of the recently
added gdb::option::filename_option_def feature.  This means that the
user now has full completion of the option names, and that file names
will still complete for the '-exec' option, but will no longer
complete if the '-exec' option is not used.

I have also converted the clone-inferior command, though this command
does not use any file name options.  This command does now have proper
completion of the command options.