Implement printf %*s and %.*s

[Crestwave]

This basically uses the first argument as a replacement for the asterisk, i.e. printf '%*s' 10 foo should be equivalent to printf %10s foo and printf '%.*s' 2 bar should be equivalent to printf '%.2s' bar.

[andychu]

OK interesting, which programs use it? That will help prioritize things, e.g. we want to fill out more of:

https://github.com/oilshell/oil/wiki/Shell-Programs-That-Run-Under-OSH

[Crestwave]

Well, um, it was used in pfetch, but I found out that it wasn't POSIX and reported it so now it doesn't. I also use it in shmenu and it's also used in fff, though.

[andychu]

#653 also mentioned printf %()T and these could be tackled together

[andychu]

cde3f10

[andychu]

Fixed: http://www.oilshell.org/blog/2020/03/release-0.8.pre3.html