Options: Correctly check for missing arguments

Relying on the value of optind for detecting missing arguments is
unreliable because its value after a failed parse is an implementation
detail. A more correct way to achieve this is to pass ':' at the
beginning of option string, which tells getopt to return ':' for missing
arguments.

For this to work, I also had to add a nullptr at the end of the argv
vector, as some getopt implementations did not work without that. This
is also an implementation detail, as getopt should normally be called
with argc+argc "as to main function" (i.e. null-terminated).

Thanks to Michał Górny for testing this patch out on NetBSD.

llvm-svn: 364317