lang: Add getenv function

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

test/shell/env0.mcl

@@ -0,0 +1,19 @@
+$tmpdir = defaultenv("TMPDIR", "/tmp")
+
+$x = getenv("TEST")
+$y = getenv("DOESNOTEXIST")
+$z = getenv("EMPTY")
+
+$a = defaultenv("TEST", "321")
+$b = defaultenv("DOESNOTEXIST", "321")
+$c = defaultenv("EMPTY", "456")
+
+$t = hasenv("TEST")
+$f = hasenv("DOESNOTEXIST")
+
+$env = env()
+$m = maplookup($env, "TEST", "321")
+
+file "${tmpdir}/environ" {
+	content => printf("%s,%s,%s:%s,%s,%s:%t,%t:%s", $x, $y, $z, $a, $b, $c, $t, $f, $m),
+}

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