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