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

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

At this time, only 'server' supported 'backup' keyword.
This patch makes also 'default-server' directive support this keyword.
A new keyword 'no-backup' has been added so that to disable 'backup' setting
both in 'server' and 'default-server' directives.

For instance, provided the following sequence of directives:

default-server backup
server srv1
server srv2 no-backup

default-server no-backup
server srv3
server srv4 backup

srv1 and srv4 are declared as backup servers,
srv2 and srv3 are declared as non-backup servers.