#1 online configuration updates

[Stuart D. Gathman]

Currently, changing configuration requires stopping and
restarting the milter. With sendmail configured to
TEMPFAIL connections while milter is down, no mail gets
lost. However, it shouldn't be that hard to support
reloading the config at runtime.

Step 1 - gather all config items into a class, have a
single reference to the config class.

Step 2 - store the current config reference in each
milter context. This way, existing connections will
use a consistent configuration.

Step 3 - provide a thread to listen for a signal or
something to load a new config object and the single
reference (which should be atomic). Existing
connections will continue to use the old config, but
new connections will use the new config.

Step 4 - be sure to log the config loading event.