copy-builtin: posix conformance

copy-builtin: posix conformance

This commits contains changes to allow running `copy-builtin` without
bash + some minor improvements.

changed shebang to /bin/sh
added -f option to `set` to globally disable unneeded globbing
replaced all `echo` commands within add_after() with `printf`
alternative to avoid possible issues with options (-neE)
dropped non-portable superfluous `readlink` command
replaced superfluous `true` command with `:` builtin alternative
replaced non-portable `--recursive` option of `cp` command with `-R`
alternative
dropped non-portable `local` keyword

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: illiliti <illiliti@protonmail.com>
Closes #12004