tool_urlglob: polish, cleanups, improvements

tool_urlglob: polish, cleanups, improvements

- assert instead of printing "internal error" for unlikely events
- avoid allocating the main struct
- convert globerror() from macro to function
- renames to shorter and clearer names
- malloc + copy => memdup0
- change buffer handling to dynbuf
- realloc to handle more globs, but use less memory for few

Closes #18198