Warn if nsslapd-idletimeout is set lower than the default or is 0

[rcritten]

nsslapd-idletimeout is used by 389 to determine when a client has gone idle. The default value is 3600. A value of 0 is unlimited.

We saw a case where a user set it to 10 which caused all sorts of issues because the client though it was still connected and reported failures as SERVER_DOWN and much later than the connection was created.

So we should WARN if the value is less than 3600 or if it is set to 0. The attribute lives in cn=config.