mhshow: Automatically invoke (one) pager for the whole message, if on TTY.

mhshow: Automatically invoke (one) pager for the whole message, if on TTY.
Mhshow used to invoke the pager for each of the text/plain parts
independently. Now, we invoke it once for the whole message.
Remember: We serialize!
(Mhl(1) invokes a pager too, currently, but not much longer.)