date: fix %N modifier handling

[dragutreis]

Fixes #11658

Fix %N (nanoseconds) strftime modifier handling to match GNU behavior:

• %_3N now correctly truncates to 3 digits and right-fills trailing zeros with spaces (e.g., "0 " instead of "0")
• %-N now correctly returns all 9 digits when no width is specified (e.g., "000000000" instead of "0")

The fix handles %N's unique semantics: width truncates from the right (not expands), _ flag right-fills with spaces, and - without width is a no-op.

Includes unit tests and integration tests.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/user. tests/misc/user is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/nl/multiple-files. tests/nl/multiple-files is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/nl/nl. tests/nl/nl is passing on 'main'. Maybe you have to rebase?
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/cp/preserve-mode is no longer failing!
Congrats! The gnu test tests/misc/selinux is no longer failing!
Congrats! The gnu test tests/tee/tee is no longer failing!
Congrats! The gnu test tests/touch/empty-file is no longer failing!
Congrats! The gnu test tests/touch/fifo is no longer failing!
Congrats! The gnu test tests/touch/no-rights is no longer failing!
Note: The gnu test tests/cp/copy-FMR is now being skipped but was previously passing.
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Note: The gnu test tests/shuf/shuf-reservoir is now being skipped but was previously passing.
Note: The gnu test tests/sort/sort-stale-thread-mem is now being skipped but was previously passing.
Note: The gnu test tests/sort/sort-u-FMR is now being skipped but was previously passing.
Congrats! The gnu test tests/printf/printf-surprise is now passing!
Congrats! The gnu test tests/seq/seq-epipe is now passing!

[sylvestre]

thanks
please also add a test in tests/by-util/test_date.rs