Fix -S with -l not being atomic.

[bdrewery]

Summary

It was unlinking the target even though it uses rename(2) which already
effectively does that. -S is intended to not unlink(2) the target first.

MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

Test Plan

Truss and:

/usr/tests/usr.bin/xinstall # kyua test
install_test:copy_self  ->  passed  [0.036s]
install_test:copy_self_comparing  ->  passed  [0.035s]
install_test:copy_self_safe  ->  passed  [0.035s]
install_test:copy_self_safe_comparing  ->  passed  [0.035s]
install_test:copy_to_nonexistent  ->  passed  [0.035s]
install_test:copy_to_nonexistent_backup  ->  passed  [0.035s]
install_test:copy_to_nonexistent_backup_safe  ->  passed  [0.035s]
install_test:copy_to_nonexistent_comparing  ->  passed  [0.035s]
install_test:copy_to_nonexistent_preserving  ->  passed  [0.035s]
install_test:copy_to_nonexistent_safe  ->  passed  [0.035s]
install_test:copy_to_nonexistent_safe_comparing  ->  passed  [0.035s]
install_test:hard_link  ->  passed  [0.033s]
install_test:mkdir_simple  ->  passed  [0.052s]
install_test:overwrite  ->  passed  [0.035s]
install_test:overwrite_backup  ->  passed  [0.036s]
install_test:overwrite_backup_comparing  ->  passed  [0.037s]
install_test:overwrite_backup_safe  ->  passed  [0.036s]
install_test:overwrite_backup_safe_comparing  ->  passed  [0.037s]
install_test:overwrite_comparing  ->  passed  [0.035s]
install_test:overwrite_eq  ->  passed  [0.035s]
install_test:overwrite_eq_comparing  ->  passed  [0.035s]
install_test:overwrite_eq_safe  ->  passed  [0.035s]
install_test:overwrite_eq_safe_comparing  ->  passed  [0.035s]
install_test:overwrite_safe  ->  passed  [0.035s]
install_test:overwrite_safe_comparing  ->  passed  [0.035s]
install_test:strip_changing  ->  passed  [0.045s]
install_test:strip_changing_comparing  ->  passed  [0.045s]
install_test:strip_changing_overwrite  ->  passed  [0.045s]
install_test:strip_changing_overwrite_comparing  ->  passed  [0.045s]
install_test:strip_changing_overwrite_eq  ->  passed  [0.045s]
install_test:strip_changing_overwrite_eq_comparing  ->  passed  [0.045s]
install_test:strip_noop  ->  passed  [0.038s]
install_test:symbolic_link  ->  passed  [0.035s]
install_test:symbolic_link_absolute  ->  passed  [0.036s]
install_test:symbolic_link_relative  ->  passed  [0.036s]

Results file id is usr_tests_usr.bin_xinstall.20160809-192916-083663
Results saved to /root/.kyua/store/results.usr_tests_usr.bin_xinstall.20160809-192916-083663.db

35/35 passed (0 failed)

[bdrewery]

These came in with the initial import of -l from NetBSD, but NetBSD never had these unlinks. The unlink is kept for the default (no -S) case.