MEDIUM: peers: add the ability to disable a peers section

MEDIUM: peers: add the ability to disable a peers section

Sometimes it's very hard to disable the use of peers because an empty
section is not valid, so it is necessary to comment out all references
to the section, and not to forget to restore them in the same state
after the operation.

Let's add a "disabled" keyword just like for proxies. A ->state member
in the peers struct is even present for this purpose but was never used
at all.

Maybe it would make sense to backport this to 1.5 as it's really cumbersome
there.