Btrfs-progs: implement new subcommand parser

Btrfs-progs: implement new subcommand parser

This completely replaces the existing subcommand infrastructure, which
is not flexible enough to accomodate our needs.  Instead of a global
command table we now have per-level tables and command group handlers,
which allows command-group-specific handling of options and subcommands.
The new parser exports a clear interface and gets out of the way - all
control over how matching is done is passed to commands and command
group handlers.

One side effect of this is that command implementors have to check the
number of arguments themselves - patch to fix up all existing commands
follows.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>