ln: Use stdbool, style nits.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: imp, allanjude
Differential Revision: https://reviews.freebsd.org/D44511

(cherry picked from commit 437d53daf71a357aae960881ef037564736f7441)

ln: Clean up and simplify tests.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44512

(cherry picked from commit e0afcbc85690b6464706ead57103baa0493fdfb2)

ln: Add a test case for ln -sfF.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: allanjude, asomers
Differential Revision: https://reviews.freebsd.org/D44513

(cherry picked from commit 2ae8d34666a6dea4c1c77a1905c7b8cebd6a21f9)