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_spec.sh

@@ -0,0 +1,73 @@
+#shellcheck shell=sh
+Include lib/core.sh
+Include lib/var.sh
+
+Describe 'lib/var.sh'
+  Describe 'myos_var'
+    It 'returns the value of a plain variable'
+      SOME_VAR=plain
+      When call myos_var SOME_VAR
+      The output should equal "plain"
+    End
+    It 'returns empty for an unset variable'
+      When call myos_var NEVER_SET
+      The output should equal ""
+    End
+    It 'returns empty for an empty name'
+      When call myos_var ""
+      The output should equal ""
+    End
+    It 'never runs a command that happens to share the name'
+      # `host` is a real program: a lazy default must never be confused with it
+      When call myos_var host
+      The output should equal ""
+      The status should be success
+      The stderr should equal ""
+    End
+  End
+
+  Describe 'lazy defaults'
+    lazy() {
+      myos_default_LAZY_ONE() { printf 'from-%s' "$(myos_var LAZY_BASE)"; }
+      myos_var LAZY_ONE
+    }
+    It 'computes the default when the variable has no value'
+      LAZY_BASE=a
+      When call lazy
+      The output should equal "from-a"
+    End
+    It 'recomputes it against the current values'
+      When run source spec/unit/var_lazy_helper.sh
+      The line 1 should equal "from-first"
+      The line 2 should equal "from-second"
+      The line 3 should equal "pinned"
+    End
+    It 'declares a default from a string too'
+      When run source spec/unit/var_default_helper.sh
+      The output should equal "built-here"
+    End
+    It 'refuses a default written in terms of itself'
+      When run source spec/unit/var_loop_helper.sh
+      The stderr should include "defined in terms of itself"
+      The status should equal 1
+    End
+  End
+
+  Describe 'myos_var_is_lazy'
+    It 'is true for a variable that only has a default'
+      myos_default_ONLY_LAZY() { echo x; }
+      When call myos_var_is_lazy ONLY_LAZY
+      The status should be success
+    End
+    It 'is false once the variable has a value'
+      myos_default_BOTH() { echo x; }
+      BOTH=explicit
+      When call myos_var_is_lazy BOTH
+      The status should be failure
+    End
+    It 'is false for a program on PATH'
+      When call myos_var_is_lazy host
+      The status should be failure
+    End
+  End
+End