cp: better permissions mode handling with umask awareness

[my4ng]

Fix #10787, #10862, Supersede #10859.

This PR reworks the permissions mode handling such that its behaviour is in line with GNU's implementation. Specifically:

• set umask to !0o077, which means (1) cp can actually create and work with new directories/files which will have 700 permissions mode before their final calculated mode is set. the original umask is kept and passed around for calculation in the case of newly created and non-mode-preserving dir/files. umask is restored to original on exit.

• for explicit non-mode-preserving, newly created dir/files, use 777 and 666 modes for calculation respectively.

• for newly created dir/files, always set their permission mode to the correct one.

• added 12 new tests for the combination of existing/not-existing, no-preserve/default/preserve, dir/file.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/cp/cp-parents. tests/cp/cp-parents is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/factor/t34 is no longer failing!
Congrats! The gnu test tests/printf/printf-surprise is now passing!

[ChrisDryden]

Is there any way to split this PR up into something smaller? When its this big it gets pretty hard to review

[my4ng]

I'm afraid not. Most changes are actually from the noise of propagating orig_umask, and the only significant changes being build_dir and handle_mode. Without either, it results in a dozen or more failed tests.

There is a GNU test regression: tests/cp/cp-parents, caused by the second to last test in that file due to created parent dir not having attributes set correctly. Will fix soon.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/pr/bounded-memory. tests/pr/bounded-memory is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/factor/t34 is no longer failing!
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!