MINOR: server: Make 'default-server' support 'disabled' keyword.

MINOR: server: Make 'default-server' support 'disabled' keyword.

Before this patch, only 'server' directives could support 'disabled' setting.
This patch makes also 'default-server' directives support this setting.
It is used to disable a list of servers declared after a 'defaut-server' directive.
'enabled' new keyword has been added, both supported as 'default-server' and
'server' setting, to enable again a list of servers (so, declared after a
'default-server enabled' directive) or to explicitly enable a specific server declared
after a 'default-server disabled' directive.

For instance provided this configuration:

    default-server disabled
    server srv1...
    server srv2...
    server srv3... enabled
    server srv4... enabled

srv1 and srv2 are disabled and srv3 and srv4 enabled.

This is equivalent to this configuration:

    default-server disabled
    server srv1...
    server srv2...
    default-server enabled
    server srv3...
    server srv4...

even if it would have been preferable/shorter to declare:

    server srv3...
    server srv4...
    default-server disabled
    server srv1...
    server srv2...

as 'enabled' is the default server state.