Allow 0 as a valid value (never close idle connections)

Increased maximum allowed value to 3600 s (1 hour)
Use 'ap_timeout_parameter_parse' to allow more flexible configuration (i.e. h, min, s, ms suffixes)
Use 'apr_time_from_sec' when applicable.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1700418 13f79535-47bb-0310-9956-ffa450edef68