Before this change, only response headers could be manipulated with the
Caddyfile's 'header' directive.
Also handle the request Host header specially, since the Go standard
library treats it separately from the other header fields...
Use piles from which to draw config values.
Module values can return their name, so now we can do two-way mapping
from value to name and name to value; whereas before we could only map
name to value. This was problematic with the Caddyfile adapter since
it receives values and needs to know the name to put in the config.