install: create destination file with safer modes before copy

[nerdroychan]

This is a followup PR to #6595.

Regarding the unconditional removal of the destination file problem, before this patch, the destination file is removed unconditionally when copy_file is called. Now a healthy check is added, which only removes the destination file if the source file exists and can be opened to read.

Furthermore, this patch uses std::io::copy with file descriptors to avoid the chmod 644 problem in the original patch. This is verified using strace.

A new test case is added as well.

Ping @mjguzik.

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)

[sylvestre]

sorry but it needs to be rebased

[RenjiSann]

ping @nerdroychan

[sylvestre]

please reopen when ready