MINOR: server: set maintenance mode on "purge server"

MINOR: server: set maintenance mode on "purge server"

This patch changes behavior of "purge server" to automatically set a
server in maintenance mode if needed. This is not performed if a server
cannot be removed due to a configuration limitation.

The objective is to improve the user API. While not a big change in the
context of a single "purge server", automatic maintenance mode will
become a must have for the future "purge all servers". Thus, this change
allow guaranteeing a similar behavior between these two commands.

To implement it, a new argument is added to srv_check_for_deletion(),
indicating the deletion mode. For better code clarity, this is defined
as an enum type pr_del_mode.