Fix remaining zgrep(1) wrapper script regressions

Summary:
Fix short flags without whitespace, e.g:

zgrep -wefoo test

Fix multiple -e flags:

zgrep -e foo -e xxx test

Previously only the last pattern would be used.

Clean up possible leading blank in ${grep_args}.

Update comment: 2.51 -> 2.6.0

Add a test case for the last known zgrep wrapper issue: recursion
(-r) not implemented.

Reviewers: markj, kevans, ngie, bapt

Reviewed By: markj

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D48873