tac: accept a separator that starts with '-'

[emirmrkaljevic]

Summary

tac -s -1 and tac --separator -1 failed with unexpected argument '-1' found, while GNU tac uses -1 as the separator. The separator argument now accepts values starting with - (allow_hyphen_values(true)), the same fix as #15345 for cut --output-delimiter.

$ printf 'a-1b-1' | ./target/debug/tac -s -1
b-1a-1

Fixes #15497

Test plan

• New test_separator_starting_with_hyphen covers -s -1, --separator -1 and --separator=-1; it fails without the change and passes with it
• cargo test --no-default-features --features tac test_tac::: 34 passed
• cargo fmt --check, cargo clippy -p uu_tac --all-targets -- -D warnings
• Output matches GNU coreutils 9.12 for all three forms

Found with behave.

[sylvestre]

Thanks for your PR