+/*
+ * This is the main entry point for command line argument parsing.
+ *
+ * Parse command line arguments according to structure options,
+ * starting at position opt_index.
+ *
+ * All output of parsed values is via pointers in options.
+ *
+ * Parsing stops at -- (consumed) or at the (k+1)st argument
+ * not starting with -- (a "positional argument") if options contains
+ * k positional argument descriptors.
+ *
+ * Returns the index of first non-parsed argument, or -1 in case of error.
+ *
+ */