close/reopen groups

[mariux]

As discussed in #22 it might be good to be able to close a group (and maybe reopen it later).

closed groups shall not accept new jobs.

[mariux]

e.g. add MXQ_GROUP_FLAGS_CLOSED to group_flags

[mariux]

Use

• mxqadmin -c <group-id> to close a group
• mxqadmin -o <group-id> to reopen a group

mxqsub honors closed state of a group.

This is implemted as a flag as proposed by @donald in #22

[mariux]

This does not affect already queued jobs in any way at the moment.