zgrep: fix exit status with multiple files

zgrep should exit with success when given multiple files and the
pattern is found in at least one file. Prior to this change,
it would exit with success only if the pattern was found in _every_ file.

Reviewed by: dab ngie
MFC after: 2 weeks
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D26616