lang: Add getenv function

$x = getenv("NAME")
    $y = defaultenv("NOTEXIST", "defaultvalue")
    $z = hasenv("NAME")
    $a = env()
    $b = maplookup($a, "NAME", "defaultvalue")

test/shell/env0.sh

@@ -0,0 +1,21 @@
+#!/bin/bash -e
+
+set -o errexit
+set -o pipefail
+
+. ../util.sh
+
+# these values represent environment variable values below or defaults set in test/shell/env0.mcl
+regex="123,,:123,321,:true,false:123"
+
+tmpdir="$($mktemp --tmpdir -d tmp.XXX)"
+
+env TMPDIR="${tmpdir}" TEST=123 EMPTY="" $timeout -sKILL 60s "$MGMT" run --tmp-prefix --converged-timeout=5 --lang env0.mcl
+e=$?
+
+egrep "$regex" "$tmpdir/environ" || fail_test "Could not match '$(cat "$tmpdir/environ")' in '$tmpdir/environ' to '$regex'."
+
+# cleanup if everything went well
+rm -r "$tmpdir"
+
+exit $e