xargs: support -i option without an equals sign before the argument

[brian-pane]

Some of the failures in the Gnu test suite happen because tests use the idiom xargs -iARG, and the Rust xargs implementation currently requires an equals sign after the -i.

As a simple example,

echo "123" | xargs -iARG echo ARG
123

echo "123" | ./target/release/xargs -iARG echo ARG
Error: error: unexpected argument '-A' found

  tip: to pass '-A' as a value, use '-- -A'

Usage: xargs [OPTIONS] [COMMAND]...

For more information, try '--help'.

For compatibility, I think the command line parser should support the use of -i without the =.

I'm willing to work on a PR for this.

[brian-pane]

I did some prototyping and found that implementing this compatibly with Gnu xargs will likely require a change to the clap library. The issue is that, while it's easy to change the arg specification to not require the =, clap will parse -i echo as being equivalent to -iecho (i.e., the -i flag with a value of echo), whereas for xargs that should be interpreted as the -x flag with no value followed by echo as the first positional parameter.