recoverdisk(8): treat output file consistently and abort on EINVAL

This improves usability a little as we no longer require using touch.
Also reword the manpage wrt. parameters and fix usage() [1]

With no media in a cd(4) drive, the reads will loop producing EINVAL,
abort in that case [2].

Document the shortcoming of sectorsize and MAXPHYS (a quick solution
to this might be having MAXPHYS as the "bigsize", in short testing it
didn't make a difference on throughput).

Submitted by:	arundel [1]
PR:		bin/154528 [2]

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