`docker network` is the second command with subcommands.
This refactoring pulls out parsing and processing of subcommands
from `docker volume` completion and thus makes its logic available
for other commands.
Also enables `__docker_pos_first_nonflag` for subcommand completion.
Signed-off-by: Harald Albers <github@albersweb.de>
Upstream-commit: 26294f39f2
Component: cli