Exclude - from user-supplied OPTSPEC for short options

[UrsaDK]

[A followup from #14]

To enable support for long options via the built-in getopts, OPTSPEC is extended with -:. However, since the built-in getopts processes OPTSPEC characters from right to left, there’s a risk that - could be redefined by the user, leading to two scenarios:

• If the user’s OPTSPEC defines -: (a dash followed by an argument), the current behavior for - is duplicated, and getopts_long works as intended.

• If the user’s OPTSPEC defines - (a dash without an argument), it overrides getopts_long’s definition (-:). This causes long options (e.g., --test) to be interpreted as a series of short options (-, t, e, s), which is definitely not the intended behavior.

The simplest solution to avoid this, is to ensure that user-supplied OPTSPEC for short options does not include the - character, and raise an error/warning if it does.

[UrsaDK]

Implemented as part #28 in this commit – 80eb03a