mkimg(1): MFC: process non-seekable output gracefully

mkimg may make severe load only to fail in the end
if output is non-seekable pipe, socket or FIFO
unless output format is raw disk image.

Check it out and fail early. Make it clear in the manual.

(cherry picked from commit 7f0dc6e2cdfa0317c9917dd46e9da9d3897a8fbb)