ln: Improve link(1) variant of ln(1).

• Give link(1) its own usage message.
• Use getprogname(3) instead of rolling our own.
• Verify that the target file does not already exist.
• Add tests specific to link(1).

MFC after: 3 days
Sponsored by: Klara, Inc.
Reviewed by: allanjude
Differential Revision: https://reviews.freebsd.org/D44635