sh: Do parameter expansion before printing PS4 (set -x).

The function name expandstr() and the general idea of doing this kind of
expansion by treating the text as a here document without end marker is from
dash.

All variants of parameter expansion and arithmetic expansion also work (the
latter is not required by POSIX but it does not take extra code and many
other shells also allow it).

Command substitution is prevented because I think it causes too much code to
be re-entered (for example creating an unbounded recursion of trace lines).

Unfortunately, our LINENO is somewhat crude, otherwise PS4='$LINENO+ ' would
be quite useful.

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