derive the certificates a server needs from what it routes

The hostnames are already declared, once, in the fabio route tags. A
domains.txt would be a second source of truth free to disagree with what is
actually served, so myos cert reads the tags instead and decides on its own
which name needs a wildcard: one is asked for where a tag uses one, and it
absorbs the concrete names it covers. A wildcard covers a single label, so
a.b.example.org keeps its own certificate.

dehydrated issues them: a shell script, no python, which fits a tool that has
to install on any server. It answers http-01 itself on a port bound to the
loopback and routed by fabio, and delegates dns-01 to a provider hook. The
deploy hook writes the two file names fabio watches for, through a temporary
name so fabio never reads half a certificate.

Fixed on the way: the port parser wanted six spaces of indentation and the
catalogue writes four, so every stack that binds its ports was reported
unbound.

spec/unit/cert_groups_helper.sh

@@ -0,0 +1,10 @@
+#shellcheck shell=sh
+# myos_cert_names normally reads the compose configuration; here it is replaced
+# by a fixed list so the grouping can be checked on its own.
+MYOS_CERT_MODE=$1
+_fixture=${2-'urlprefix-ipfs.ex.org/*
+urlprefix-*.ipns.ex.org/*
+urlprefix-a.ipns.ex.org/*
+urlprefix-ex.org/*'}
+myos_cert_names() { printf '%s' "$_fixture" | myos_cert_parse; }
+myos_cert_groups