fix regression in getopt_long support for non-option arguments

fix regression in getopt_long support for non-option arguments

commit b72cd07f176b876aa51864d93aa8101477b1d732 added support for a
this feature in getopt, but it was later broken in the case where
getopt_long is used as a side effect of the changes made in commit
91184c4f16b143107fa9935edebe5d2b20bd70d8, which prevented the
underlying getopt call from seeing the leading '-' or '+' character in
optstring.

this commit changes the logic in the getopt_long core to check for a
leading colon, possibly after the leading '-' or '+', without
depending on the latter having been skipped by the caller. a minor
incorrectness in the return value for one error condition in
getopt_long is also fixed when opterr has been set to zero but
optstring has no leading ':'.