[sebxt]

A non-integer value for --threads, --subsample or --cpumask made the vmaf tool abort in error() with Assertion failed: (long_opts[n].name) instead of printing usage.

These cases passed 't', 's' and 'c' to parse_unsigned(), but the long options are registered with ARG_THREADS, ARG_SUBSAMPLE and ARG_CPUMASK, so error() could not find the option in long_opts. Pass the ARG_* values, as --gpumask and --frame_cnt already do.

After the change:

$ vmaf -r ref.y4m -d dis.y4m --threads abc
Invalid argument "abc" for option --threads; should be an integer

Testing: added test_invalid_unsigned_option_prints_usage to test_cli_parse. It runs cli_parse() in a child process, since usage() exits, and checks for exit status 1. It fails on master (the child aborts) and passes with the fix. It is skipped on Windows, where fork() is not available. ninja -C libvmaf/build test passes (28/28).

Fixes #1694