prep to improve do_submit coverage

will need at least two tests (-A int/-A ext) mirroring
the two do_fetch tests being moved.
do_submit: both codepaths tested
fetch supports -o options
clarify use of -A, -o options

* -o may be used more than once
* -o, -A may be used to submit *and* fetch
clarify default value for $pastish_user
manpage: fix .Lk macro use