DAOS-18595 gurt: Handle "ERR" and "DBUG" environment values with DD_STDERR

[knard38]

Description

Previously, the DD_STDERR environment variable did not support the same set of values as DD_LOG_MASK.
This patch updates the management of DD_STDERR to ensure consistent value handling between both environment variables.

Steps for the author:

• Commit message follows the guidelines.
• Appropriate Features or Test-tag pragmas were used.
• Appropriate Functional Test Stages were run.
• At least two positive code reviews including at least one code owner from each category referenced in the PR.
• Testing is complete. If necessary, forced-landing label added and a reason added in a comment.

After all prior steps are complete:

• Gatekeeper requested (daos-gatekeeper added as a reviewer).

[github-actions]

Ticket title is 'ERROR and DBUG debug level names can not be used with DD_STDERR env variable'
Status is 'In Progress'
https://daosio.atlassian.net/browse/DAOS-18595

[daosbuild3]

Test stage NLT completed with status UNSTABLE. https://jenkins-3.daos.hpc.amslabs.hpecorp.net/job/daos-stack/job/daos//view/change-requests/job/PR-17759/1/testReport/