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/unit/var_default_helper.sh

@@ -0,0 +1,3 @@
+#shellcheck shell=sh
+myos_default BUILT 'printf "built-%s" here'
+myos_var BUILT