Fix zgrep --version

"zgrep --version" is expected to print the version information in the
same way as "zgrep -V". However, the case handling the --version flag
is never reached, so "zgrep --version" prints:

zgrep: missing pattern

instead of:

grep (BSD grep, GNU compatible) 2.6.0-FreeBSD

Reviewed by: yuripv
Approved by: yuripv (src)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29813