MINOR: server: Add 'no-agent-check' server keyword.

MINOR: server: Add 'no-agent-check' server keyword.

This patch adds 'no-agent-check' setting supported both by 'default-server'
and 'server' directives to disable an agent check for a specific server which would
have 'agent-check' set as default value (inherited from 'default-server'
'agent-check' setting), or, on 'default-server' lines, to disable 'agent-check' setting
as default value for any further 'server' declarations.

For instance, provided this configuration:

    default-server agent-check
    server srv1
    server srv2 no-agent-check
    server srv3
    default-server no-agent-check
    server srv4

srv1 and srv3 would have an agent check enabled contrary to srv2 and srv4.

We do not allocate anymore anything when parsing 'default-server' 'agent-check'
setting.