Show error if first arg doesn't start with -
Print command-line option where parsing failed
cmdline: Tell if there are too few / too many args

Fixes: bug #495
cmdline: Allow combining short options

Fixes: bug #496
cmdline: Add support for custom arg names
cmdline: Use "DIR" as arg name for path args
cmdline: Use "LEVELS" as arg name for --debug
Update the `arx` man page
Update the `arxsavetool` man page
Update the `arxunpak` man page
Add a "Usage" line to the help output
Fix return code for cmdline errors