bsdconfig: add datetime module for live system clock

[dteske]

Summary

Provide bsdconfig datetime (menu), date, and time to set the running
system clock via dialog(1)/bsddialog(1) --calendar and --timebox with
mustberoot. Unlike bsdinstall's time helper, this operates on the live
system rather than a CHROOT-staged install target, and calls
adjkerntz(8) after a successful change so the CMOS stays in sync.

[dteske]

Part of my work in releasing the window manager that I built for the touch-sensitive Framework Laptop 12. My window manager has a start bar complete with battery level and system date/time. I programmed it so that when you click on the clock in the start bar that it brings up this, bsdconfig datetime, so that the date and time can be set more conveniently. As mentioned elsewhere, you can't use the bsdinstall module for that purpose.

[bcr]

OK for the doc/man page change.

[dteske]

In D58487#1350655, @bcr wrote:

OK for the doc/man page change.

Thank you!