printf: support %q

[Luv-Ray]

fix #5468
Now printf %q '"$test"' can produce the right result:

./target/debug/printf %q '"$test"'; echo
\"\$test\"

However I have a doubt about the description in GNU manual:

An additional directive %q, prints its argument string in a format that can be reused as input by most shells. Non-printable characters are escaped with the POSIX proposed $'' syntax, and shell metacharacters are quoted appropriately. This is an equivalent format to ls --quoting=shell-escape output.

And in my local:

$ ls --quoting=shell-escape
'"$test"'
$ printf %q '"$test"'; echo
\"\$test\"

What does "equivalent format" mean?

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/touch/relative is no longer ERROR!

[tertsdiepraam]

That is indeed curious. This does seem to give the same output as ls. I think echo might be doing the escaping in your example:

env printf %q '"$test"'

I'd like this to use the same function as in ls because we have a pretty good robust implementation of it there. The implementation of it is here: https://github.com/uutils/coreutils/blob/main/src/uucore/src/lib/features/quoting_style.rs

Note too that there is a little spelling error (the word "metacharacters" needs to be ignored).

[Luv-Ray]

Sorry for the mistake, actually I'm not using the GNU's printf when I directly call printf, which has made me confused.

$ printf %q '$'; echo
\$
$ /usr/bin/printf %q '$'; echo
'$'
$ env printf %q '$'; echo
'$'

There is an echo because printf doesn't add an \n at the end of the output, and It's not the cause of the difference between commands.

[tertsdiepraam]

Looking good, thanks! Accidentally using the shells printf happens to me all the time too 😄

[Luv-Ray]

This unnecessary push cause wrong error message: printf: %7bb: invalid conversion specification\n, which has one more b.

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/rm/rm2 is no longer failing!
Congrats! The gnu test tests/tail/inotify-dir-recreate is no longer failing!
Skip an intermittent issue tests/rm/rm1
GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?