zgrep: fix "binary file matches" mislabeling

zgrep: fix "binary file matches" mislabeling

Problem reported by Jim Avera (Bug#31280).
This became more of an issue when GNU grep 3.5 (2020) started sending
"binary file matches" diagnostics to stderr instead of to stdout.
* tests/Makefile.am (TESTS): Add zgrep-binary.
* tests/zgrep-binary: New test.
* zgrep.in (args): New var, to accumulate args separately
from grep command, so we can prepend args if need be.
Most uses of 'grep' changed to use 'args' instead, or also.
(with_filename): Set to 1 if more than one file and -h not given;
this simplifies later code.
(gnuish_grep): New var; evaluates to true if grep supports
-H and --label options, as is true for GNU and FreeBSD grep.
Append -H to 'grep' if outputting file names with GNUish grep,
and use --label with GNUish grep unless reading from stdin,
as that’s safer and more efficient than relabeling with 'sed'.