date: Add support for nanoseconds

This patch introduces support for a conversion specification for
nanoseconds.

The format of %N is meant to be compatible with that of GNU date.

The nanoseconds conversion specification is implemented directly in
date(1) instead of libc (in strftime(3)) to avoid introducing
non-standard functions to libc at this time and modifying struct tm.

Apart from introducing the nanoseconds conversion specification, this
patch brings the following changes:

• The "ns" format for ISO 8061 dates is now unlocked. E.g., date -Ins prints: 2024-04-22T12:20:28,763742224+02:00
• The -r flag when fed a file is now aware of the nanosecond part of the last modification time.
• date(1) is now able to set the time with nanosecond precision. It is not possible as of now to do that by specifying nanoseconds directly via the command-line arguments. Instead, the -r flag can be used.
• date(1) is now using the clock_gettime(3) family of functions instead of ctime(3) family of functions where possible.

Reviewed by: des, markj
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D44905