ln: avoid unlinking destination before hard-link source validation

[can1357]

uutils ln -f unlinks an existing destination before attempting a hard link, so a missing source deletes the destination even though link creation fails.
GNU ln attempts linking first and only removes and retries when replacement is actually possible.

Reproduction Steps

touch /tmp/ln_dst && ln -f /tmp/nonexistent_source /tmp/ln_dst
# Expected (GNU): reports missing source, prints exit=1, and still lists /tmp/ln_dst
# Actual (uutils): reports hard-link creation failure, prints exit=1, and /tmp/ln_dst is missing

Impact

ln -f can silently destroy existing files when the source path is wrong or disappears, breaking GNU compatibility and causing data loss in scripts that rely on force-replace semantics.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/pipe-f is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.
Congrats! The gnu test tests/seq/seq-epipe is now passing!