builtin: Ignore first -- in getopts per POSIX
Issue 7, XCU, getopts, OPTIONS reads "None.",
and getopts isn't a special built-in listed in sexion 2.14 ‒
this means that XCU, 1. Introduction, 1.4 Utility Description Defaults,
OPTIONS, Default Behavior applies:
Default Behavior: When this section is listed as "None.", it means
that the implementation need not support any options. Standard
utilities that do not accept options, but that do accept operands,
shall recognize "--" as a first argument to be discarded.
Test with: getopts -- d: a
Correct output is no output, exit 1
Wrong output errors out with d: being an invalid argument name
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>