The package used a lot of string-formatting, followed by string-splitting.
This looked to originate from attempts to use templating to allow future
extensibility (9a3ab03).

Looking at the history of the package, only a single update was made to
these templates, 5 years go, which makes it unlikely that more templating
will be needed.

This patch simplifies the handling of arguments to use `[]string` instead
of a single `string` (and splitting to a `[]string`). This both simplifies
the code somewhat, and prevents user/group-names containing spaces to be
splitted (causing, e.g. `getent` to fail).

Note that user/group-names containing spaces are invalid (or at least
discouraged), there are situations where such names may be used, so we
should avoid breaking on such names.

Before this change, a user/group name with a space in its name would fail;

    dockerd --userns-remap="user:domain users"
    INFO[2020-08-19T10:26:59.288868661+02:00] Starting up
    Error during groupname lookup for "domain users": getent unable to find entry "domain" in group database

With this change:

    # Add some possibly problematic usernames for testing
    # need to do this manually, as `adduser` / `useradd` won't accept these names
    echo 'user name:x:1002:1002::/home/one:/bin/false' >> /etc/passwd; \
    echo 'user name:x:1002:' >> /etc/group; \
    echo 'user name:1266401166:65536' >> /etc/subuid; \
    echo 'user name:1266401153:65536' >> /etc/subgid; \
    echo 'user$HOME:x:1003:1003::/home/one:/bin/false' >> /etc/passwd; \
    echo 'user$HOME:x:1003:' >> /etc/group; \
    echo 'user$HOME:1266401166:65536' >> /etc/subuid; \
    echo 'user$HOME:1266401153:65536' >> /etc/subgid; \
    echo 'user'"'"'name:x:1004:1004::/home/one:/bin/false' >> /etc/passwd; \
    echo 'user'"'"'name:x:1004:' >> /etc/group; \
    echo 'user'"'"'name:1266401166:65536' >> /etc/subuid; \
    echo 'user'"'"'name:1266401153:65536' >> /etc/subgid; \
    echo 'user"name:x:1005:1005::/home/one:/bin/false' >> /etc/passwd; \
    echo 'user"name:x:1005:' >> /etc/group; \
    echo 'user"name:1266401166:65536' >> /etc/subuid; \
    echo 'user"name:1266401153:65536' >> /etc/subgid;

    # Start the daemon using those users
    dockerd --userns-remap="user name:user name"
    dockerd --userns-remap='user$HOME:user$HOME'
    dockerd --userns-remap="user'name":"user'name"
    dockerd --userns-remap='user"name':'user"name'

Signed-off-by: Sebastiaan van Stijn <[email protected]>