[Bug 247126] [PATCH] zgrep(1) does not handle -f FILE correctly

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=247126

            Bug ID: 247126
           Summary: [PATCH] zgrep(1) does not handle -f FILE correctly
           Product: Base System
           Version: 12.1-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Only Me
          Priority: ---
         Component: bin
          Assignee: bugs at FreeBSD.org
          Reporter: leres at freebsd.org

Somewhere between FreeBSD 11 and 12, zfgrep and friends became a wrapper shell
script. We've found that it does not handle the -f flag correctly:

    # Works
    % zfgrep RELEASE /etc/motd
    FreeBSD 12.1-RELEASE-p3 GENERIC 

    # Hangs reading from stdin
    % echo RELEASE > /tmp/0
    % zfgrep -f /tmp/0 /etc/motd
    /etc/motd:FreeBSD 12.1-RELEASE-p3 GENERIC
    ^Z
    Suspended
    leviathan 89 % ps t
      PID TT  STAT    TIME COMMAND
      11249  4  Ss   0:00.11 -csh (csh)
      12190  4  T    0:00.00 /bin/sh /usr/bin/zfgrep -f /tmp/0 /etc/motd
      12191  4  T    0:00.00 /usr/bin/zcat -f -
      12192  4  T    0:00.00 grep -F -f /tmp/0 -- /etc/motd -
      12193  4  R+   0:00.00 ps t

The problem is when -f is used there is no command line pattern and the wrapper
script incorrectly uses the first non-flag argument as a pattern and
incorrectly decides we want to read from stdin. Meanwhile grep is invoked with
the pattern (really the target file) and a '-'. In then outputs any matches and
hangs, reading from stdin.

Here's the most minimal patch I could develop in two minutes.

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