This also cleans up the code by using @Type(), eliminating the need
for the argFlag() and boolFlag() functions.
Allowing [:0]const u8 arguments makes this parser useful for
river-control commands as well.
This is currently unused and I don't like the approach anymore
regardless. If/when we need positional arguments (probably when
implementing the upcoming river-control protocol in rivertile)
they should be handled separately from flags.
This commit also improves the CLI error reporting to always print the
usage string if invalid arguments were passed.
