tr: some small improvements

[cakebaker]

This PR applies five small improvements:

• removes a spell-checker:ignore line
• sets "after help" in uu_app instead of uumain
• merges some imports
• uses a logical AND (&&) instead of a bitwise AND (&)
• uses sets_len == 1 instead of sets_len < 2 for better readability because in our case sets_len can never be 0

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)