Support storing UnixFS 1.5 Mode and ModTime

[kstuart]

Feature Progress

• Can ipfs add with preserved mode and/or last modification time
  • on files
  • on directories
• Can ipfs add with custom mode and/or last modification time
  • on files
  • on directories
• Can ipfs get restoring mode and/or last modification time
  • on files
  • on directories
  • in archives
• Can ipfs files chmod to change mode
  • on files
  • on directories
• Can ipfs files touch to change last modification time
  • on files
  • on directories
• Automatically update the last modification time when file data is changed or truncated (e.g. ipfs files write)
• Can add files and directories with mode and/or modification time using multipart-form data
• ipfs files stat reports mode and last modification time

Related PRs

• support file mode and last modified time for ipfs add interface-go-ipfs-core#66
• Support for storing file mode and modification times go-unixfs#85
• initial support for Mode and ModTime on Node interface go-ipfs-files#31
• initial support for setting/updating file modes and modification times go-mfs#93
• support restoring permissions and last modification times tar-utils#11

Implementation Notes

• When adding files and directories without opting to store a mode or modification time the same CIDs are generated that would
  have been created before this feature was implemented (opt-in).
• The Go runtime currently has no native support for restoring file mode and modification time on symbolic-links, support for r
  estoring the last modification time has been added for Linux distributions and the following BSDs: freebsd, netbsd, openbsd, dr
  agonflybsd.
• Automatically updating a modification time will only occur if a modification time was previously stored.
• When creating an archive, for compatibility, time resolution is to the second; Nanoseconds are not supported.

IPFS Add (CLI)

The ipfs add options --preserve-mode and --preserve-mtime are used to store the original mode and last modified time of the file being added, the options --mode, --mtime and --mtime-nsecs are used to store custom values, a custom value of 0 is a no-op as is providing --mtime-nsecs without --mtime.

The preserve flags and custom options are mutually exclusive, if both are provided the custom options take precedence.

---

Closes #6920

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