periodic: Fix periodic reports when log files are not compressed.

The modern zcat(1) is capable of handling compressed and uncompressed
text files, so we can simply use zcat command.

PR: 253168
Reviewed by: delphij
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43357

(cherry picked from commit 1fa4ddcc6de6a0c46416f719a5b7efa1169f51ce)