flag.sh: Add ability parse combined short flags

[nhooyr]

like -bfzs

Should be parsed as 4 separate flags, not one whole bfzs.

I'm thinking I add a function to iterate through each character somehow and if they don't recognize one of the characters as a flag or it wasn't a single hyphen flag, then we error out.

[nhooyr]

Maybe flag.sh should be changed to use getopts in combination. Perhaps long flags could be supported by a separate optstring.

This would also solve parsing of flags like -oarg where there is no separator between the flag and the argument.