Resolve "'nextpart' is not compatible with 'set -x'" [v9_18]

If 'set -x' is in effect file.prev gets populated with debugging output. To prevent this open descriptor 3 and redirect stderr from the awk command to descriptor 3. Debugging output will stay directed to stderr.

(cherry picked from commit 10f67938)

Closes #3630 (closed)

Edited by Mark Andrews

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