keep the dynamism of make in pure shell

Two mechanisms, matching what the make engine actually did:

Lazy defaults. A stack setting is a function myos_default_<VAR>, called only
when the variable has no value, and called again at every reference. That is
exactly a recursive ?=: an explicit value wins, and the default follows a
DOMAIN that a .env changes later. The prefix is what makes it safe; the first
version used a bare function named after the variable, and the test suite
caught it running /usr/bin/host for a stack group called host.

Templates. myos env-update fills a .env from the .env.dist files, expanding
${VAR} against the current values and running $(command), forward references
included.

Also fixed: the project .env now wins over /etc/conf.d/myos, which is what the
documentation claimed and the code did not.

share/make/shim.mk lets a project keep make as a front end: every myos command
becomes a target that shells out to bin/myos, and the project keeps its own
targets and its stack .mk files. It sits outside make/ because the legacy
engine globs every .mk in there.

spec/golden/expected/shim-config.txt

@@ -0,0 +1,2 @@
+docker compose -f @WD@/stack/host/consul.yml -f @MYOS@/share/compose/networks.yml -p testhost --project-directory @WD@/stack/host config
+[exit 0]

spec/golden/expected/shim-env.txt

@@ -0,0 +1,2 @@
+COMPOSE_PROJECT_NAME testhost
+[exit 0]

spec/golden/expected/shim-project-target.txt

@@ -0,0 +1,2 @@
+would renew the certificates of host/consul host/fabio
+[exit 0]

spec/golden/expected/shim-up-group.txt

@@ -0,0 +1,4 @@
+docker network create tester-local
+docker network create testhost
+docker compose -f @WD@/stack/host/consul.yml -f @WD@/stack/host/fabio.yml -f @MYOS@/share/compose/networks.yml -p testhost --project-directory @WD@/stack/host up -d
+[exit 0]