Support "mhng-show --stdout"

This prints raw to stdout, instead of running through less.  This is
useful when debugging with GDB as less doesn't play nice with that.
Work around some Krste's mail client bugs

Krste's client sends UTF-8 characters marked as US-ASCII, which is
illegal.  This works around the bug by just never passing messages
marked as US-ASCII to iconv, because US-ASCII is a subset of UTF-8,
which means the conversion will at best do nothing (and at worst fail
on invalid US-ASCII).