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.

share/make/shim.mk

@@ -0,0 +1,57 @@
+##
+# myos, from make.
+#
+# This file gives a project the myos commands as make targets, without the make
+# engine: every target shells out to bin/myos, which is pure POSIX sh. What the
+# project keeps from make is what make is actually good at, and the CLI is not:
+# its own targets, its own dependencies, and the .mk files of its stacks.
+#
+#   MYOS ?= /usr/local/lib/myos
+#   include $(MYOS)/share/make/shim.mk
+#
+# It lives outside make/ on purpose: the legacy engine includes every .mk of
+# that directory, and would pull this one in too.
+#
+# Then `make up STACK=host` and `myos up host` do the same thing, through the
+# same code. Variables given on the command line are forwarded, so
+# `make up STACK=host DOMAIN=example.org` behaves as expected.
+
+MYOS                            ?= $(patsubst %/share/make/shim.mk,%,$(lastword $(MAKEFILE_LIST)))
+MYOS_BIN                        ?= $(MYOS)/bin/myos
+STACK_DIR_NAME                  ?= stack
+STACK_DIR                       ?= $(wildcard $(CURDIR)/$(STACK_DIR_NAME))
+
+# variable MYOS_ARGS: variables set on the make command line, forwarded to myos
+MYOS_ARGS                       ?= $(foreach v,$(MAKEOVERRIDES),$(v))
+
+.DEFAULT_GOAL                   := help
+
+## the stack files of the project may add their own targets and variables
+include $(wildcard $(STACK_DIR)/*.mk $(STACK_DIR)/*/*.mk)
+
+# function make: run a myos command, for the stack .mk files that call it
+define make
+	$(MYOS_BIN) $(MYOS_ARGS) $(1)
+endef
+
+# target help: List the myos commands
+.PHONY: help
+help:
+	@$(MYOS_BIN) help
+
+# target myos: Run an arbitrary myos command, as in `make myos ARGS="up host"`
+.PHONY: myos
+myos:
+	@$(MYOS_BIN) $(MYOS_ARGS) $(ARGS)
+
+# make tries to remake every makefile it read, and the catch-all below would
+# hand each of them to myos as a command. An empty rule stops that.
+$(MAKEFILE_LIST): ;
+
+# target %: Hand anything else to myos
+## a target the project defines itself keeps precedence over this rule
+%: FORCE
+	@$(MYOS_BIN) $(MYOS_ARGS) $@ $(ARGS)
+
+.PHONY: FORCE
+FORCE: ;