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/fixtures/shim-project/Makefile

@@ -0,0 +1,2 @@
+MYOS ?= /usr/local/lib/myos
+include $(MYOS)/share/make/shim.mk

spec/fixtures/shim-project/stack/host/consul.yml

@@ -0,0 +1,12 @@
+services:
+  consul:
+    image: hashicorp/consul:1.15
+    container_name: ${HOST_COMPOSE_PROJECT_NAME:-localhost}-consul
+    network_mode: host
+    restart: always
+    environment:
+      CONSUL_HTTP_TOKEN: ${HOST_CONSUL_HTTP_TOKEN}
+    volumes:
+      - consul:/consul/data
+volumes:
+  consul:

spec/fixtures/shim-project/stack/host/fabio.yml

@@ -0,0 +1,13 @@
+services:
+  fabio:
+    image: fabiolb/fabio:1.6.3
+    container_name: ${HOST_COMPOSE_PROJECT_NAME:-localhost}-fabio
+    depends_on: [consul]
+    ports:
+      - "80:80"
+      - "443:443"
+    environment:
+      FABIO_REGISTRY_CONSUL_ADDR: ${DOCKER_HOST_INET4:-127.0.0.1}:8500
+      FABIO_LOG_ACCESS_TARGET: ${HOST_FABIO_LOG_ACCESS:-}
+    networks:
+      - public

spec/fixtures/shim-project/stack/host/host.mk

@@ -0,0 +1,7 @@
+host ?= host/consul host/fabio
+
+# a target of the project, on top of the myos commands: this is what make is
+# kept for, and what the shim leaves alone
+.PHONY: host-certs
+host-certs:
+	@echo "would renew the certificates of $(host)"

spec/fixtures/shim-project/stack/host/registrator.yml

@@ -0,0 +1,9 @@
+services:
+  registrator:
+    image: gliderlabs/registrator:master
+    container_name: ${HOST_COMPOSE_PROJECT_NAME:-localhost}-registrator
+    network_mode: host
+    depends_on: [consul]
+    command: -internal=false -useIpFromLabel SERVICE_ADDRESS consul://127.0.0.1:8500
+    volumes:
+      - ${DOCKER_SOCKET_LOCATION:-/var/run/docker.sock}:/tmp/docker.sock