date(1): Add "-R" flag to use RFC 2822 date and time output format [MFC]

As stated in the man page, this is equivalent to use "%a, %d %b %Y %T %z"
as the output format while LC_TIME is set to the "C" locale.

This is compatible with date(1) from the GNU core utilities.

This is an MFC of r264968 and r264970.