add basic webserver that can do most of what i need

- serve static files, serving index.html or optionally listings for directories
- redirects
- reverse-proxy, forwarding requests to a backend

these are configurable through the config file. a domain and path regexp have to
be configured. path prefixes can be stripped.  configured domains are added to
the autotls allowlist, so acme automatically fetches certificates for them.

all webserver requests now have (access) logging, metrics, rate limiting.
on http errors, the error message prints an encrypted cid for relating with log files.

this also adds a new mechanism for example config files.

doc.go

@@ -41,6 +41,7 @@ low-maintenance self-hosted email.
 	mox config domain rm domain
 	mox config describe-sendmail >/etc/moxsubmit.conf
 	mox config printservice >mox.service
+	mox examples [name]
 	mox checkupdate
 	mox cid cid
 	mox clientconfig domain
@@ -388,6 +389,12 @@ date version.
 
 	usage: mox config printservice >mox.service
 
+# mox examples
+
+List available examples, or print a specific example.
+
+	usage: mox examples [name]
+
 # mox checkupdate
 
 Check if a newer version of mox is available.