MINOR: stats: also support a "no-maint" show stat modifier

MINOR: stats: also support a "no-maint" show stat modifier

"no-maint" is a bit similar to "up" except that it will only hide
servers that are in maintenance (or disabled in the configuration), and
not those that are enabled but failed a check. One benefit here is to
significantly reduce the output of the "show stat" command when using
large server-templates containing entries that are not yet provisioned.

Note that the prometheus exporter also has such an option which does
the exact same.