last: Mild style cleanup

[des]

Summary

• Prefer explicit NULL comparisons to using pointers as booleans
• Prefer explicit 0 comparisons to using strcmp() as a predicate
• Avoid use of strcpy()

Sponsored by: Klara, Inc.

[markj]

usr.bin/last/last.c

513

Not a problem in practice, but it's not very nice that this helper function sometimes returns a malloc()ed buffer, and sometimes not.