httpd: fix SIGHUP by invalidating cache on reload

httpd: fix SIGHUP by invalidating cache on reload

Since we require separate PublicInbox::HTTPD instances for each
listen socket address (in order to support {SERVER_<NAME|PORT>}
for PSGI env), the old cache needed to be invalidated on rare
app refreshes.

SIGHUP has always been broken in -httpd (but not -imapd or
-nntpd) due to this cache.

Update the daemon documentation and 5.10.1-ize some bits while
we're in the area.