Simplify cache inode expire configuration

Rather than have two parameters, just have a single parameter.

< 0 means "never"
= 0 means "immediate"
> 0 means "expire after the indicated time"

Note that large values may be practically the same as "never"...

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Add CONF_ITEM_I32_SET config item type

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>