tzsetup(8): update paths to zone tab and tzdata source

tzsetup now uses zone1970.tab and zoneinfo source is located in
contrib/tzdata, update the man page accordingly.

While here, drop obsolete Tn macros and insert vertical breaks (mostly
before Dl in examples) to improve readability.

Reviewed by: pauamma_gundo.com (manpages)
Differential Revision: https://reviews.freebsd.org/D39643