configmain: Update paths to default paths

Suggesting "/usr/local/nagios" as a good default value is silly.

Signed-off-by: Robin Sonefors <ozamosi@flukkost.nu>
configmain: Remove deprecated/deleted keys

As these keys have never worked in any stable Naemon release, they
should be gone and never mentioned again.

Signed-off-by: Robin Sonefors <ozamosi@flukkost.nu>
configmain: Add missing sections, update texts

The "update" include removing misguided, hyperbolic panic, suggesting
that precache isn't really very good, and saying the command pipe is
sucky compared to the query handler.

Signed-off-by: Robin Sonefors <ozamosi@flukkost.nu>