flag: add ability to define short, long flags and combine short flags

Modify library to distinguish between long and short flags.
for example flag_int should have the following signature:

long *flag_long(flag_Parser *p, char *long_name, char *short_name, long default_val, char *usage);

Where long_name and short_name can be NULL or empty (\0) to have the short or long flag undefined)
When the flag is a short flag, the parser should allow using multiple short flags together so -f and -a could be used together like this: -fa (this matches the behavior of getopts)

Start by locating the existing flag_int declaration and the flag_Parser implementation in the library. Trace how flag names are defined and parsed, then verify the requested API accepts optional long and short names and that short flags can be combined such as -fa. Done means both naming forms and combined short flags work without breaking existing parsing behavior.