install: ancestor directories respect process umask instead of using fixed 0755 (diverges from GNU)

Description

When install -D or install -d creates ancestor directories, it passes DEFAULT_MODE=0755 to the underlying mkdirat/mkdir syscall, which still has the process umask applied by the kernel. GNU install instead calls umask(0) at startup, so ancestor directories are always created with exactly 0755 regardless of the caller's umask.

Impact

For any umask that has bits set within the 0755 pattern — e.g. 0027, 0077, 0111 — uutils creates ancestor directories with a different mode than GNU:

Note: umask 0002 (the trigger for issue #11363) coincidentally does not change 0755 because it only strips world-write, which 0755 does not have — so that specific report was caused by a broken test rather than this bug. However, the underlying behavioral divergence is real.

GNU behavior

From install.c in GNU coreutils:

• main() calls umask(0) early, before any file operations
• make_ancestor() calls mkdir(component, DEFAULT_MODE) where DEFAULT_MODE = S_IRWXU | S_IRGRP | S_IXGRP | S_IROTH | S_IXOTH (0755)
• Because umask is 0, ancestor directories always get exactly 0755

Affected code

• src/uu/install/src/install.rs: both code paths (directory() line ~498 and standard() line ~687) create ancestor directories without zeroing umask first
• src/uucore/src/lib/features/safe_traversal.rs: create_dir_all_safe documents that "the actual mode will be modified by the process umask"

Fix

Zero the process umask early in install's uumain (unix only), matching GNU's approach. uucore::mode already uses rustix::process::umask internally for get_umask(); a zero_umask() helper can be exported from there.

Related: #11363, PR #12713

Start at src/uu/install/src/install.rs, especially uumain and the directory() and standard() paths, then read the umask handling in src/uucore/src/lib/features/safe_traversal.rs and uucore::mode. Verify the install behavior under umasks such as 0027, 0077, and 0111; done means ancestor directories match GNU's 0755 behavior on Unix without changing unrelated paths.