calendar: use iconv to respect the output encoding

calendar(1) can have input in various encoding, specifying
LANG=<locale_name> to enable calendar(1) to determine which one to use.

The problem is the content of the calendar itself is exposed as is making it
unreadable in many cases. For example french calendar which is encoded
ISO8859-1 is rendered badly in a fr_FR.UTF-8 environment.

Using iconv allows to solve this issue.
This will also allow to keep only 1 encoding in base for those files without
breaking user existing setup

Reported by: many
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D19221