chasquid: Dovecot support (experimental)

This patch adds dovecot support to the chasquid daemon, using the
internal dovecot library added in previous patches.

Dovecot support is still considered EXPERIMENTAL and may be reverted, or
changed in backwards-incompatible ways.

The patch also adds the corresponding integration test, which brings up
a dovecot server with a custom configuration, and tests chasquid's
authentication against it.  If dovecot is not installed, the test is
skipped.

internal/config/config.go

@@ -93,4 +93,6 @@ func LogConfig(c *Config) {
 	log.Infof("  Suffix separators: %s", c.SuffixSeparators)
 	log.Infof("  Drop characters: %s", c.DropCharacters)
 	log.Infof("  Mail log: %s", c.MailLogPath)
+	log.Infof("  Dovecot auth: %v (%q, %q)",
+		c.DovecotAuth, c.DovecotUserdbPath, c.DovecotClientPath)
 }