add initial support for commit notifications to gotd(8) At present only email notifications are implemented. Code for HTTP notifications is not yet finished, hence HTTP-related documentation remains hidden for now. This adds a new 'notify' process which has an "exec" pledge. It runs helper programs which implement the notification transport layer, such as got-notify-email which speaks SMTP. This design avoids having to link all of gotd with network libraries and related crypto libraries. Notification content is generated by the 'repo_write' process. Commit log messages and diffstats are written to a file which the 'notify' process will pass on to its helpers on stdin. The default output looks similar to 'got log -d'. If too many new commits are present the output looks similar to 'got log -s' instead. Tags always look like 'got tag -l'. The session process coordinates generation of notifications. It maintains a notification queue which holds one notification per updated reference, and passes notification requests from this queue to the 'repo_write' process for notification content creation and then to the 'notify' process for notification delivery. Only one notification can be in flight at a time to avoid file descriptor starvation if many references get updated in a single client session. ok op@

remove the gotsh group requirement from gotd; any user can now connect Repository access is now controlled by access rules in gotd.conf, and concurrent connections to the gotd socket by local users are limited by the listen process. We should keep refining our anti-DoS measures in the future, but at least we have something in place now. ok jamsek, op

fork gotd repo_read/repo_write children on demand ok op, jamsek

change example to use useradd; ok stsp@

add a test suite for gotd(8); check basic clone and send functionality