MEDIUM: server: properly support and propagate the maintenance status

MEDIUM: server: properly support and propagate the maintenance status

This change now involves a new flag SRV_ADMF_IMAINT to note that the
maintenance status of a server is inherited from another server. Thus,
we know at each server level in the chain if it's running, in forced
maintenance or in a maintenance status because it tracks another server,
or even in both states.

Disabling a server propagates this flag down to other servers. Enabling
a server flushes the flag down. A server becomes up again once both of
its flags are cleared.

Two new functions "srv_adm_set_maint()" and "srv_adm_set_ready()" are used to
manipulate this maintenance status. They're used by the CLI and the stats
page.

Now the stats page always says "MAINT" instead of "MAINT(via)" and it's
only the chk/down field which reports "via x/y" when the status is
inherited from another server, but it doesn't say it when a server was
forced into maintenance. The CSV output indicates "MAINT (via x/y)"
instead of only "MAINT(via)". This is the most accurate representation.

One important thing is that now entering/leaving maintenance for a
tracking server correctly follows the state of the tracked server.