When the --keyring option was added, the `,` between `mirror:` and
`keyring:` was omitted in the long option string passed to `getopt`.
This causes `--mirror` to be parsed as `--mirror:keyring`, matching
`*) break ;;` and prematurely terminating option parsing. To avoid
this, add the missing `,`. Also order `keyring:` before `mirror:` to
preserve lexical ordering of long options.
Fixes: eebcd76feb ("lxc-debian: allow using a custom keyring")
Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
