getopt: Add support for :: optional arguments

Adds support for getopt extension: option characters followed by
two colons. A character followed by a single colon indicates
that an argument is to follow the option on the command line.
Two colons indicates that the argument is optional (this is
transparent extension not covered by POSIX).

optsting = "a::b:X"

In the string above, option a will accept an optional argument.
(Option requires an argument) Usage:

Option syntax	Meaning
-a              OK, No argument provided (optional).
-afoo           OK, argument is foo
-a foo          Wrong, no space allowed with optional arguments.
                foo is considered a non-option argument.
-bfoo           OK, argument is foo (required).
-b foo          OK, argument is foo (required).
-b              Wrong, option b requires an argument.

Since the argument is optional, you will have to check the value
of optarg to see if it is a valid pointer (otherwise, it's NULL).

JIRA: RTOS-511

unistd/getopt.c

@@ -74,6 +74,10 @@ int getopt(int argc, char * const argv[], const char *optstring)
 			/* Argument in the same argv */
 			optarg = &argv[optind][optwhere];
 		}
+		else if (*(++optspec) == ':') {
+			/* Optional argument */
+			optarg = NULL;
+		}
 		else if (optind + 1 < argc) {
 			/* Argument in the next argv */
 			optarg = argv[optind + 1];