security: Validate and disallow tasks that have the name "alloc"

[jrasell]

If a task is named "alloc" the filesystem security guarantees for the allocation are broken. It allows any other task in the allocation to access the task directory for the task named "alloc". This is because the impacted tasks directory name mimics the allocation directory name which all tasks have access to.

Fix this by disallowing "alloc" as a name for a task and return an error to the job submitter.

Links

Closes #26999
Jira https://hashicorp.atlassian.net/browse/NMD-1056

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[tgross]

LGTM