logging: add DirMode options and propagate FileMode to rotations

[DeRuina]

Summary

This PR enhances the FileWriter logging module to give users explicit control over directory creation permissions and ensures that rotated log files inherit the correct file mode from configuration. Closes #7314

Changes

• Added DirMode option to FileWriter:

  • "inherit" → copies the nearest existing parent directory’s permissions, normalizing r→x for directories.
  • "from_file" → derives directory permissions from the file’s mode (e.g., 0644 → 0755, 0600 → 0700).
  • Octal strings (e.g., "0755") → directly specify directory permissions.
  • Default remains 0700 (same as before, ensuring backward compatibility).

• Improved permission handling:

  • When DirMode is set, directories are created using the requested mode.
  • Added normalizeDirPerm() to ensure directories with read bits also gain execute bits (so they’re traversable).

• Propagated file mode to timberjack:

  • Added FileMode: os.FileMode(fw.Mode) to the embedded timberjack.Logger.
  • Ensures rotated log files now use the same mode as the original log file.

• Security-conscious defaults:

  • Maintains restrictive defaults (0700 directories, 0600 files).
  • Umask still applies.
  • r→x normalization applies only to directories — not to files.

Tests Added

• Extended test coverage in:

  • filewriter_test.go (Unix)
  • filewriter_test_window.go (Windows)

• Added cases for:

  • DirMode values: inherit, from_file, explicit octal, and default.
  • Directory creation under different umasks.
  • Correct parsing of dir_mode from the Caddyfile.
  • Windows smoke test to ensure successful file creation across modes.

Assistance Disclosure

AI was used to generate this PR summery and also to expand some tests I wrote. Everything was reviewed by me.

[CLAassistant]

All committers have signed the CLA.

[francislavoie]

Thanks! Please don't forget to sign the CLA

[DeRuina]

I just signed 👍

[francislavoie]

Sorry for the delay! Looks great to me. I added another commit to fixup an issue with roll_at and roll_minutes config parsing I noticed while writing the docs for those options, so I'm sneaking that in here while I'm at it.