Remove dubious workaround in parse_unary().

The comment "eat first char of illegal label, else endless loop on
implied .WORD" was no longer true. It did reveal some other small
gotchas, which are corrected now as well.

tests/SetTestOk

@@ -0,0 +1,21 @@
+#!/bin/sh
+#
+set_ok() {
+    t="$1"
+
+    if [ -e "$t".lst.ok ]
+    then
+        cp "$t".lst "$t".lst.ok
+    fi
+
+    if [ -e "$t".objd.ok ]
+    then
+        cp "$t".objd "$t".objd.okd
+    fi
+}
+
+for fn in "$@"
+do
+    set_ok "$fn"
+done
+