common/btrfs: rename and enhance _require_btrfs to _require_btrfs_command

common/btrfs: rename and enhance _require_btrfs to _require_btrfs_command

Rename _require_btrfs() to _require_btrfs_command() to avoid
confusion, as all other _require_btrfs_* has a quite clear suffix,
like _require_btrfs_mkfs_feature() or _require_btrfs_fs_feature().

Also enhance _require_btrfs_command() to accept 2nd level commands
or options.

Options will be determined by the first "-" char.  This is quite
useful for case like "btrfs inspect-internal dump-tree" and "btrfs
check --qgroup-report".

[eguan: make local vars in _require_btrfs_command "local"]

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>