tzsetup: default to symlink

[emaste]

Summary

Use a symlink for /etc/localtime unless it already exists and isn't a symlink.

[jrm]

I, perhaps naively, still prefer your approach in https://reviews.freebsd.org/D37005. However, I may not appreciate valid cases where /usr might not be mounted. This is certainly a step in the right direction.

[emaste]

Doing something like this change (or D37004) now doesn't preclude D37005 from going in later.

[emaste]

D37005 committed