mailinfo: Do not use -u=<encoding>; say --encoding=<encoding>

mailinfo: Do not use -u=<encoding>; say --encoding=<encoding>

Specifying the value for a single letter, single dash option
parameter with equal sign looked funny, and more importantly
calling the flag to override encoding from utf-8 to something
else "-u" (obviously abbreviated from "utf-8") did not make any
sense.  So spell it out.

Signed-off-by: Junio C Hamano <junkio@cox.net>