Opt: Simpified and corrected processing of OPT_SINGLE

Opt: Simpified and corrected processing of OPT_SINGLE

OPT_SINGLE is now checked in opt_parse_value(). Previously, short,
long, and positional options had their own checks.

OPT_CL_SWITCH did not allow changing the value of the switch once
it is set, although documentation in opt.h claimed quite the opposite.
Changes are currently allowed unless OPT_SINGLE is set.

Test cases have been updated accordingly.