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.

lib/cmd/env-update.sh

@@ -0,0 +1,26 @@
+#shellcheck shell=sh
+# myos env-update  fill the .env of the workdir from the .env.dist it finds
+#
+# Templates are read from the workdir and from every requested stack, so a
+# stack can ship the variables it expects and their defaults.
+myos_cmd_env_update() {
+  _target=${ENV_FILE:-$WORKDIR/.env}
+  _dists=$MYOS_ARGS
+  if [ -z "$_dists" ]; then
+    for _ref in $MYOS_STACKS; do
+      _d=$(myos_stack_resolve "$_ref" 2>/dev/null) || continue
+      for _c in "$_d/.env.dist" "$_d/$(myos_stack_name "$_ref").env.dist"; do
+        [ -f "$_c" ] && _dists="${_dists:+$_dists }$_c"
+      done
+    done
+    for _c in "$WORKDIR/.env.dist" "$WORKDIR/.env.example" "$WORKDIR/.env.sample"; do
+      [ -f "$_c" ] && _dists="${_dists:+$_dists }$_c"
+    done
+  fi
+  [ -n "$_dists" ] || { myos_warning "no .env.dist found for $MYOS_STACKS"; return 0; }
+  for _dist in $_dists; do
+    myos_info "env-update $_target from $_dist"
+    myos_env_update "$_target" "$_dist" "$WORKDIR/.env.$ENV"
+  done
+  printf '%s\n' "$_target"
+}